presumably, doing the kritzkast.
yoshi, a page or two ago, I linked to a config that lowers settings and and reduces amount of in-game assets. Try seeing if that makes the game ugly enough to work on your system. If it doesn't, you're out of luck.
Hydro is a map everyone hates.
The points are all really wide and they're connected by a series of very small, very tight tunnels. Even with the signs telling you where to go, it can get a bit confusing and it's easy to get separated from your team.
And then the design itself is built to...
try using max framerate config. makes the settings low and minimizes the amount of stuff around. makes it able to play on computers that wouldn't normally be able to.
Degreaser has quick weapon switch, allowing the pyro to get burst damage through flaregun and axetinguisher. Degreaser pyros don't care about afterburn.
Phlog has MMMMPH!. Backburner has higher flame damage and crits from behind.
Regular flamethrower is solid, but both of the pyro playstyles...