for people in trouble with challenge mode, get high fire rate and damage. Try for 15 fire rate and 12 damage, then go all damage. And lookout on the flags in the middle, thats a trap =p
all levels are easy, the only hard thing is not getting hit on bonus. Boss 9 and 10 on bonus are though to not get hit. Very good game. Shields and rockts rlz :] 5/5
Score - 35420; Good game, addictive. Maxed everything except souls. Quite easy when you have shit load of money. Make more levels with mobs that walk and chase.