I found a glitch. If you do not purchase one of your allies, e.g. do not buy the flamethrower upgrade, your allies are invincible. Then you only have to worry about which allies you pick up and making sure your leader doesn't get hit.
BennyGreen and Krolan got it right. I maxed out arrow, shotgun, and canon, repeatedly alternating between weapons shot monsters down much quicker. On the ground, I used 1 frost ward, 1 row spikes, 2 rows more frost ward, 1 row barriers, 3 rows of archers. There are clearly multiple winning solutions!
 "Toxers"){kind=icon}
 "Battalion Commander 2"){kind=icon}
 "Siege Knight"){kind=icon}
 "We Become What We Behold"){kind=icon}