I'm not saying he's going to outperform say a carnosaur in melee. But he would easily be able to keep up with a lot of the heroes on foot, even if they're dedicated melee ones like a sunblood or darkoath chieftans. Hell even something like a orruk MegaBoss on foot only barely outdamages him on average. Especially when taking into consideration his sturdiness he should be useable in the front line. Plus it makes his positioning for his spells easier anyway. I'm curious if it'd actually work.Ehh, on average he gets 7 attacks, 4 hit, 3 wound at -1, 1 dmg each. That's not particularly threatening. At the end of the day he's still a spellcaster first, since for summoning you just would take a regular Slann.
Going summoning requires sacrificing your spells to gather conjugation points, so it's usually great rememberer.
Gives great board control, and you can use it twice on the same unit if the first attempt fails.