Check the "Wave Info" in the bottom-right corner. For example, if a wave deals Magic damage, your Natural armor units will be weak against it.
If you want, I can:
: Fragile but high damage. Place them behind tanks or to the side to "flank" so they aren't targeted first. Aura Buffs legion td guide full
: An excellent early-game tank with health regeneration that allows you to push workers safely. Bazooka/Pyro Check the "Wave Info" in the bottom-right corner
There is no single "best" unit, but there are dominant combos. if a wave deals Magic damage