Situationally, Watcher. Who maybe can be a topic for another day.
Overall, I have to weigh the annoyance factor of adding more AI capable mana drainers into the game. “Highest mana” would mean you could influence them away from your generator, which is good, and his damage is still rather pitiful, so I don’t think there would be too much danger there at endgame when Spirit Fox is far more annoying anyways. I still think he should have a Daemon damage conditional. So maybe change to target highest mana like you suggested, keep the full mana drain, and deal double damage to daemons (doesn’t need triple if it is also a drain). This would make him a fairly good Famine counter as well.
Regarding “highest mana”, assuming they are tied, would it also go for the first troop with that much mana, last troop with that much mana, or a random one? First troop having that mana amount is the most intuitive and probably the most useful overall, but last one makes more sense thematically with his spell name.