There’s a problem with the troops who summon other troops, it seems. I’ve definitely noticed this with the Summoner and others. Here are screenshots for the Dark Master in the Arena.
His spell card says “summon a Level 12 Thrall”…
…but he actually summons a Level 15 Thrall.
Edit: sorry, I’ve just noticed that you can’t see the actual level of the Thrall in my second screenshot, because of the spell card showing first. ^^ I’ve finished that battle already, so can’t take another one just yet, but I guarantee you he’s Level 15. There was no one to buff up his life or armor in the line-up, and he has 16 and 2 respectively.