This is a great game. I've spent many hours trying to beat all the levels on "story challenge." There's one problem though: aliens picking up power cores from their fallen brethren is kind of random.
If I build a tower that's shooting at core-bearing aliens, I run the risk of killing one right when there's a tough boss ready to carry it off. It's unpredictable, so it kind of ruins the strategy aspect. Frequently, winning one of the tougher levels is due to a guy being killed just after passing the tough alien by luck/timing and not by solid planning.
Maybe the aliens could pass off the cores when they contact a more robust comrade? Allowing them to backtrack to steal cores would work too. Both of those solutions would make things harder, but the number/strength of the aliens could be reduced for balance.