There are a couple of things:
1) Of course, I'm hesitant to attack first. I have no idea what kind of attacks he has; he might have something very powerful. I would prefer to attack after our tanks, but I guess there's no choice.
2) Such a boss might have area-of-effect attacks, fireballs, and other things. It's assumed that he fights against a large number of opponents. That is, ideally, we need to avoid clustering too much (although after everyone engages in melee, we will have to anyway). I suggest that mages be on different sides of the enemy, and warriors... Well, somehow, I don't know.
3) By the way, the mage Nebka should not only attack the enemy but also heal us. I mean, with his healing spell.
I'm also curious if there is any mind control/summoning that could level the playing field... We need to keep that in mind. /