For the Krapped On medal, buy the level two armor, the one that costs $10.000 and go against the first boss. Keep shooting him in place (have at least a decente gun and rocket for this) and when his first rapid fire ends, you'll have have have enough life to survive a couple more shots, but he'll probably be dying already. You can also do this for easy and fast cash.
PWJP
for bosses, use rockets and gunz at the same time.
also, for Kataklism, just play level 1 again.
ya feel like playing GTA now too?
HeavyTank
haha, yeah, and l happen to have GTA 2 on my old laptop :D