I admit I'm no expert , but I tend to play as whatever the team is low on, except for demoman or sniper because I'm bloody hopeless at them, lol.
I'm not much better at spy, but I've managed to really annoy the enemy engis, and I generally swap to spy when the other team decides they really need FOUR damn engineers on their team.
Just running around a tightly packed area full of sentries, dispensers etc, just going sapsapsapsapsapsap then running away....then back and going sapsapsapsap again... half the time I'll only destroy a couple of things, but I've managed to annoy them, lol.
I also tend to try to distract the enemy too, I figure if I can get 3 people trying to kill me, that's our team one down, and their team 3 down. Profit!
I'm sure as I play more, I'll learn more and get better, anyways.
My tip for achievements, to get the pyro's 'Kill someone with a taunt', get around the back in 2fort and do it behind a sniper
