As far as the challenge goes all i did was play
the 40 wave on easy and by the 27th wave or so you
achieve 25000 points or more. Easier doing it that
way than continuing to play the 20 waves over and
over again
Mixing a few archers and a few more dexers gives a
pretty solid defense against air, and a decent
amount of ground units. put a few levels into
specialist for both
Try to get at least one of each color out as early
as possible. This will allow them to gain
experience so that as you need various colors, you
can just individually upgrade them as appropriate.
You don't want to have to add a fresh color
for the first time on a wave that needs it.