As far as I observed in my own fights, it's random who gets targeted first by the boss. He puts out stun on all exept one of your team, that one gets true fear. In my fights, the boss allways hits the feared champ first. 

For targeting, the AI takes into account if a champ is injured allready and may be killed from a hit. So normally the boss should hunt his initial target for some more turns, trying to kill that champ. He may change the target of his attacks from the first champ to Toragi if the latter falls to low on hp from his ally protect.

Maybe you could try to wait with the protect and don't use it at the first turn possible, so the damage is better spread between the initial target (who would take the first hit to 100%) and Toragi. 

Be aware that this boss ignores def. Your champs need a high hp pool to survive, while def is only important if you get hit by the waves in stage 1 or 2.