Originally Posted by HungryHunter

Can't really think of some in terms of "efficient" I can think of useless units. Avoid units with cannon stage abilities: Iris and Gardenia

That's not the first time I read that, and honestly I don't understand why there are so many people saying Cannon stage ability is useless/bad. Sure, it depend entirely on the map: if there is no cannon that's useless, indeed. But note on map with cannons the ability affect ALL your 5 Teams and is additive (1 ability = 2 shoots, 2 abilities = 3 shoots etc). Much more efficient than a flat +x% Attack/Defense, especially for beginners who don't have 4 strong Teams. Even a weak Team can kill a stronger Boss who has already lost X0% of its Life.
IMO the only completely useless abilities are the [Raid Boss] ones, because Raid Boss are weak and we prefer to poke (more or less hard) them most of the time (else we share them to random players and they will kill them for us very quickly).