There are also other challenges on the older levels. There's a interesting "poison core" mode, where the core will kill any enemy it passes when it's going back to the AI, and you get no resources. This basically amounts to making sure that it doesn't get taken at all, as the lack of resources would hurt you in the long run.
There's also a super hard "Super Grinder" mode. I love Grinder modes because it's just walkers over and over again, but these throw out other aliens for 99 rounds. I was trying it out on the Last Stand level, and my current strategy isn't working. Stuff like the fast moving aliens take a big chunk out of me. (Even with lasers, it is hard to kill these guys.)
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.