Maybe at early to mid game with most of his damage coming from a static boost, but at endgame pvp he can’t oneshot that many troops even with maxed giant/barrier boost (which is incredibly unpractical to achieve to begin with).
Add event-bonuses and GW-stat bonuses and he doesn’t dream of oneshoting anything.
There are no other two good Giants, there is Jarl Godking Firemantle and then there is the weak pathetic rest of the bunch. Jarl is also the only Giant reasonably teaming up with Jotnar because he can kinda feed him mana, but even between those two there is no further synergy, with Jotnar dealing true damage, the only Giant at all that does, and Jarl not.
Anyways having more than 2 Giants makes the team clunky, slow and makes it bothersome to spread his traits barriers around.
That is really the problem with many of the people that say he is fine, they just assume so cause “omg look at that theoretically big true damage number!”. They don’t actually spent hours building teams with him and feeling how much of a clunky slow mess those teams become when you try to optimize the giant-based part of his spells boost, or how little use one relatively big true damage nuke is when there are no other true damage dealers in the team to finish the job.
Which is not remotely good for the manacost and far from oneshotting any troop in competitive play.
Spirit Fox, an UR troop, does that amount of true damage for less mana while also draining 14 mana in addition. If you account for Empowered Spirit Fox even does that damage with less than half the mana Jotnar needs for it, that is awefully inefficient for a base mythic.
If you actually build a balanced team around him that has a good manaflow to feed him, and enough 4-match opportunity to spread barriers that means leaving at least 5-10 of his damage boost behind, making his spell quite meh, effectively dealing less damage per mana and time than troops like Spirit Fox, who also has the massive added utility of manadrain.
His trait however is really good for Guild wars, it is his only redeeming factor, but a base mythic that has to sit in the back and spread barrier and shut up to never use his waste of a turn-ending spell is not my Definition of fine.