If you're a loner or your family/friends are crap at games, an easy way to beat the co-op medal is to just shoot down everything (blue missiles give you exp) but don't buy anything until around level 16-18, then you can buy everything at once :)
There's a bug in life stealing. Vampiric attacks add the normal amount of health to the attacker even if they only do 1 damage (i.e. it takes the value from the attack's hit amount, not the defender's life points)
Just completed it on hardest difficulty... wow that boss almost owned me! >_< Very nice game, needs splash damage on the rockets and the ability to select a primary target for all your weapons. And more weapons :)
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