Gas Bandit's Guide to Berserking -
You're gonna find a lot of Berserker guides out there that'll tell you that Berserkers are gods, and others will tell you they're borderline at best. I'm here to tell you they're good, but situational. They are unparalleled soloers, and great at chewing through hordes of trash mobs, but at higher difficulties and in certain "flavor" rounds (Boom!, Up Up and Decay) they are at a major disadvantage.
It's tempting to go off and do your own thing, even when playing on a server with other people, but if you're not supporting your group they might die, and what they leave behind might be something you can't do by yourself. So it's best, I find, to use your 'zerker in a protective capacity. Try to watch your buddies' flanks and deal with anything that comes up on them, and help them take down the big guys when they show up. In certain situations, you can also bodily block a chokepoint so your friends can shoot past/over you with impunity, though at later stages and higher difficulties, you might need a lot of healer support to do this.
What weapons to buy and when
My preferred upgrade path goes like this -
If you're playing with me, or if you own Road Redemption, you can buy the Road Redeemer club. This club is basically a blunt version reskin of the Katana. If you can afford a Katana or Road Redeemer at the end of the first round on short (or second round on normal, or third round on long), then get it. I like the RR better than the Katana because, although it is slightly slower, it also has greater stumbling power, which means you can use it to bat around large groups of trash mobs and keep them from attacking or getting past you. Also, Fleshpounds are resistant to slashing damage, which makes the RR do more damage to them than the Katana. But if you can't get the RR, the Katana is still an excellent weapon. The katana's "bash" attack (default key V) is actually a forward stab, not a bash, and is stupidly strong.
However, by the boss wave (5 on short or on endless), you need to have the Eviscerator. If you aren't making money like you think you should be, stick to the crovel until you can afford the eviscerator. The crovel is actually a really good weapon, probably the best "starter" class weapon of any perk. It can one-shot a lot of weaker enemies, especially if you get headshots with it (and indeed, some people swear by spending the money to upgrade the crovel instead of buying a Katana). So, it's entirely within the realm of feasibility to stick with the crovel until you have your eviscerator. The Eviscerator's M1 attack fires sawblades that can ricochet and can be retrieved when they finally stop, and the M2 attack uses gasoline to rev up the sawblade and do lots of melee damage fast. When you run out of gasoline, M2 will do the eviscerator's bash attack, which isn't as good (but better than nothing)... so usually, when I run out of gas, I switch back to my RR/Katana for melee, only taking the Eviscerator back out if I need ranged damage against Scrakes/Fleshpounds/Bosses. Or I go find more ammo boxes, as each one will also refill 50 gasoline.
Other weapons - forget the knife, you should at least always have a crovel to use in melee. The nailgun is nearly useless, only really good if you are in a boom round, and even then, you don't know it's coming until it's too late to buy anything. The zweihander and the Pulverizer are decent alternative options if you prefer to stick and move with heavy attacks instead of wading in with fast attacks. The bone crusher and static strikers are excellent soloing weapons, but their weight means you can't have an eviscerator along with them, which means I almost never get them - the eviscerator is too useful.
Grenades - the EMP grenades do almost no damage, but they de-rage, confuse, and silence mobs. If your group is getting swarmed, an EMP grenade can save the day by breaking up the pack and letting your friends escape. Sirens can't scream, husks can't flame, bloats can't puke, and Scrakes and FPs can't rage while under the effect of the EMP grenade, and although they can still melee attack, they also tend to wander in random directions. This is a huge tactical get-out-of-jail-free card, so don't be shy about tossing one when things start getting thick.
Now let's talk perk configuration.
Level 5 - Dreadnaught vs Skirmisher -
Go Skirmisher for the HP regen and speed boost. No contest. That regen is what will keep you alive and tanking, and the speed boost is important for kiting when things go to shit.
Level 10 - Vampire vs Butcher -
Vampire can seem tempting, but it's only 4hp per kill.... which means killing that fleshpound still only gets you 4hp, same as killing a clot. Basically, this means you get heals when you don't need them and don't when you do. Go Butcher instead. 20% bonus damage and 25% bonus attack speed is huge.
Level 15 - Resistance vs Parry -
A lot of people pooh-pooh resistance as being only for noobs, but if you're protecting a group with your body, it really helps to have that bonus sonic and poison resist so you can deal with the bloats and ESPECIALLY the sirens without backup. A puked teammate is largely a useless teammate, and Resistance also removes the "can't see shit" factor of getting puked on. A couple sirens can end your whole group since they like to hide behind other zeds and don't need line of sight to do their dirty work - AND they ignore armor. Dealing with Sirens fast is definitely a priority for me, and Resistance really lets me get in there and do that without fear. That said, if you are REALLY good at parrying, Parry can extend your lifespan, especially against Scrakes and FPs. But I usually go resistance.
Level 20 - Smash vs Massacre -
This depends on your playstyle and your role. Massacre will help you deal with huge amounts of trashmobs quickly, which can help you keep the area clear so you and your team can move about freely - and 9 times out of 10, whenever I've experienced a wipe, it's been because we got boxed in by too many bodies. Smash, however, is much more useful for dealing with big enemies - especially since it applies to the launched sawblades from your eviscerator. So, it's up to you - Massacre will let you take that RR/Katana out and clear a room fast as hell (I swing 3 to 4 times per second with massacre, and two headshots kills a siren that way, and the first one stops her scream to begin with), so I think a lot of people make a mistake to dismiss it in favor of smash. Consider what you usually find yourself doing in your role, and pick accordingly. Bear in mind if you use the Pulverizer hammer, the Smash bonus only applies to the blunt damage, not the explosive.
Level 25 - Rage vs Spartan -
No contest. Spartan. Being able to move and attack at normal speed during a slow-mo is a god-tier ability.
Finally, let's cover some Techniques You Should Know And Use:
1) Directional attacks.
The direction of your melee swing depends upon the direction you are moving when you attack. Moving forward means you will swing overhead, which gives you the best chance to hit the head. Moving side to side will make you swing the same direction as you are moving, which is good for stumbling and if you are lucky, get multiple headshots at once on mobs of similar height. Attacking while moving backwards will swing upwards from low, which has a good chance to cause a stumble, letting you get away from whatever you're backing away from. However, in most situations, I'm holding W while swinging, because those melee headshots are TO DIE FOR. You will one shot most trash mobs, silence and stumble most intermediate zeds, and often also stumble the big guys, giving you a second or so to get 2 or 3 more hits in before you have to dodge his counterattack. Make the most of your tactical options.
2) Parrying
Using middle mouse button with any melee weapon will attempt to "parry" with it. If you hit parry within a half second before the enemy's attack lands, you will stagger them backwards and reduce the amount of damage you take by a lot depending on perk config and weapon being used (up to 70% with the eviscerator). This is especially useful for de-raging scrakes and fleshpounds (and ESPECIALLY King Fleshpound) because the parry will still count as a "hit" and they will come back down out of rage mode despite having done very little damage. Parrying is key to controlling the flow of combat and surviving against the big boys, so it is worth taking the time and effort to get good at doing it.
3) Animation Cancelling
There are a lot of animations that can be cancelled early so you can chain into other attacks fast. My favorite one is, using the Eviscerator, when you have to reload the blades, as soon as the mag is "clanked" onto the gun, right click to start buzzsawing. This will cancel the reload animation almost a full second early, but after you've already gotten "credit" for the reload. This means you can start doing damage and protecting yourself and your friends much faster. You can also use "reload" (r) on a melee weapon after a heavy attack (right click) to cancel the animation and then immediately heavy attack again (cancelling the bash animation) to get a two-for-one heavy headshot if you're moving forward while you do it.
4) The Eviscerator combo attack
Even though the eviscerator's right click attack is a quick succession of small hits, it can also stumble the big guys. This comes in handy against scrakes epecially, as I find them hard to parry because their attack timing isn't telegraphed as clearly as the fleshpounds. A scrake will often die to just two eviscerator blade shots to the head, but the first one will send him into a rage, meaning the second blade will miss as often as not as he flails around. Instead, what I like to do is line up that first headshot and fire, then immediately charge in with the right click to buzzsaw him, staggering him out of his rage, and then when he recovers, fire another blade as the denouement right into him at point blank range, then start backing up. If he survives that, one more blade will usually finish him off, and you've just killed a scrake while taking no damage.
Boss Strategies:
In all boss fights, you want to be the one holding the boss' attention because you can take hits and regen better than any of your comrades. But that doesn't always mean staying in melee range. Here's some specifics:
Hans Volter:
His melee attacks CANNOT BE PARRIED, and the metal on his body makes buzzsawing/slashing attacks less effective than normal. The blade shots will be your primary damage here, but if he turns away from you and you've used up all the blades in your magazine, it may be better to buzzsaw him anyway just to get his attention back on you instead of letting him focus your friend while you reload. Soon as he turns back to you, then you can reload. Try not to stand in the grenades when he starts throwing them - the gas grenades aren't much to worry about if you have Resistance, but the explosive ones can end you quick.
The Patriarch:
Sorry, you're a ranged class now. Eviscerator blades only, preferably from behind cover. The patriarch deals too much damage for you to try to facetank him, or even be caught out in the open - his minigun will shred you bad, and his rockets can end you instantly. Keep running around corners every time he stops to fire, then shoot him in the head with your sawblades. If you run out of blades, go find more instead of switching to melee.
King Fleshpound:
It's your time to shine. KFP telegraphs all his attacks like western union, so parrying him is a breeze. Keep shooting him in the glowing chest module and walking backwards to kite him. When he goes into a rage, get him to attack you and then parry the hit, then go back to shooting him in the chest. If he fires his laser, stop moving and hit C to crouch - the laser will go right over your head, doing nothing. It's easy as hell to keep him focused on you like this, which gives your friends free reign to either shoot him from the back, or deal with the quarterpounds he summons. If you run out of blades, keep doing the same with the buzzsaw melee attack. If you run out of gas, go ahead and switch to your RR/Katana and keep his attention. Keep parrying those enraged attacks and ducking the lasers, and you'll be fine.
The Abomination:
Not as bad as the patriarch, but still pretty dangerous. You're gonna have to keep out of melee range because you don't want to get sucked in and eaten. His poison bombs are enough to even worry a resistance berserker, so try not to take too much of that, but often you can save a teammate by throwing yourself in front of a charging snotling so it detonates on you instead of them - you can soak the hit and keep fighting - it will incapacitate a non-berskerer with blindness and hurt them grievously. Get as many headshots on him as you can with your eviscerator's sawblades, and his head armor should melt really fast. Keep hitting him where his armor is not, and he'll go down pretty fast.