A huge problem with the one-shot spells is that they are way overcosted by virtue of being one shots, and not because of the relative strength of the spell. Tassarion’s would be broken if allowed to cast multiple times, though, because of the uncapped magic scaling, free color targeting + arcane would allow him to infiniloop to however much magic you need for a team wipe every time. He’s basically a wasted slot (for anything other than testing uncapped gem spawn mechanics) unless he at least increases the effectiveness of other troops by a third. While functionally, you can achieve this with 3 magic in some cases (boosting a spell to 2 shot vs 3 shot, or 3 shot vs 4 shot), it becomes less and less likely and on fewer troops the more you progress into the game.
At this point, I’d give him a very simple buff - instead of increasing the magic of all allies by 3, have it be 3 boosted by their current magic (3:1 boost ratio). That would make him one of the better guaranteed magic buffers in the game, while still having the the strong drawback of the one-shot and a wasted slot afterward. Enough to at least justify him taking a spot in the team in some instances.