If you haven't been able to beat it I've learned a few things after beating it (and losing at it a few too many times)... Save your missles/machinegun for the wizards, don't bother with that on the giant ogre. There does seem to be a limit to the waves that attack, at least it did stop for me during my last run at it. Also having someone with the ability to self revive helps a lot so you don't have to keep restarting if everyone dies.