I think if he was castable over and over he could have been useful — I’m thinking of him as a Shadowblade that could have been boosted by Champion of Gaard and/or Holy St. Astra.
I was thinking that, if a spider could be one-shot and then the spell was instantly reusable, then spiders would have to spawn at a rate of more than 1 per round or else Gruz would, inevitably, be able to reliably clear all the spawns without making more show up (like AoE damage does).
But you’re probably right—he’s just not good enough to even really consider for inclusion in the META, because he just needs too much help to be worth anything and doesn’t help with board manipulation at all.
Not to mention being limited to mono-type human.
On a separate could-have-been-cool note: I think it would have been really interesting for him to, upon death, kill a random enemy. Give the “Undefeated” name some meaning (because no one would win on a 1-on-1, but could only ever draw). Then the 8 mana back could just have been a standard part of his spell—cast once, never be below 8 mana again 
But I’m not in charge, so no new (potentially really annoying) META defense mechanic for us 
