I say....
Have one mage on high ground. He has 1 HP/2 Magic/1 Chain.
On either side of him are two archers(with 1 HP/1Arrow/1Range for the inner ones, the outer have no upgrade.) Then put three knights in the row ahead(of the mage and inner archers) and have the middle one have 4 HP/1 Sword/2 Shield and have the outer ones have 2 HP/1 Shield.
Tips:
A)Keep the mage on high ground
B)Make the archers hit ones closest to them in their column
C)On second thought, Make everybody hit the ones closest first.
D)Don't make the mage attack in the middle column. Like this, it will only hit the targeted and the one on the direct right(if any).
E)Attack from behind, with side exposed, or at point blank range(1 square away).
Hope this helps!
You should be able to tell your foot soldiers to attack strongest, or attack one with most health. Also, a stage to test how good towers and upgrades are in a place with infinite lives.