For me, the lack of the trinity makes things more realistic. If there are 4 people in a room beating on seriously big dude. Does anyone really think that he will stand there and hit the biggest, strongest one carrying all of the heavy armor and ignore the others? He will pick on the smallest, weakest one and wipe them out first.
Running away is the soundest strategy for the small guy he is chasing.
Trinity basically just gives people jobs to do in the fights. Make sure you have enough people of each type for each job.
What would be really cool is if the bosses could learn from the fights. Players just use AOE and he starts to avoid it. Players hit him and then hide behind a pillar. He destroys the pillar.
I remember a karate game that did that when I was younger. If you kept doing round house kicks, the boss would start blocking them.