I have gotten to wave 38 in endless, what has worked best is leveling up life, infection chance, and speed to level ten as fast as you can, then catching attack and defense up and keeping attack pretty high. Neither attack nor defense has a level cap but speed life and infection chance max out at 20. Also the puker, fat guy, nuke, and extra infection chance should e picked up whenever you get the option, even early before you have invested a lot of points in your skills, putting bacon down and one of the green splat things on too of it is funny awesome, they all run to eat the bacon and then turn to zombies from the goo, use a nuke when you get multiple heros in a round and try to have four anti heros at all times, even if they suck you can throw one out there for a round with one annoying human left rather than waste a nuke or something good, and last thing I always get one purchase of grenades, usually after the first round, and I use them to take out the last annoying human if your zombies can't get them, by round ten or fifteen though just use the grenades up because they don't do enough damage to justify wasting a slot with them.
