Personally I've only ever seen him on super niche arena teams, where he runs completely counterintuitive to his design - people gear him with high ATK and RES, so that his passive prevents death. The RES prevents the unkillable from being removed. Then, when he does get his turn, the high ATK couples with his A3 to provide essentially an 11X AOE nuke, which is absolutely lethal. Problem, though, is that there are a ton of counters to that strategy, which is why he is pretty niche.

He could also serve as an AOE DEF-up champ for CB, since his A2 is on 3-turn CD unbooked. 

On top of other potential applications, I could see Black Knight doing work as a Magma Dragon specialist with that A1.