I'd say Scorpion Trap! My survivor is currently Level 24 and there's nothing one or two scorpions didn't manage to kill so far. I actually stopped using them because the game became too easy. Is a scorpion actually supposed to kill endless numbers of enemy units? Is there no end to the scorpion menace?
Poison Weapon is another very strong skill. With my weapon coated in poison i kill MOST units i've encounted so far with 1 slash, very useful indeed.
Any other "imbalances"? <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />