I think one of the strongest possibly unintended features of summoners in Arena is that summoning a unit allows your team to gain an extra turn, allowing a first round Chronos Distortion or similar.This is actually a very powerful mechanic as having more turns than your opponent allows you to tick down your cool downs and execute skills while your opponent has no chance to react.
I would suggest putting summoned units under a "free time" category, meaning when summoned units act they will not cause skill cool downs on either team to tick.
If a similar fix is not implemented I fear 3 summoner is going to be the only arena build because of the massive advantage in skill tempo.