Relying on npc to beat a boss instead of yourself is kinda pathetic, but also thats not the real issue.

The real issue is how the bosses can be sometimes unkillable because some skills can make them able to regain health fast or be able to deal unavoidable attacks.
I once fought a boss that that had way too high health regen that even if you tried to attack them they would gain it too quickly to almost never be able to beat them, while they also had projectile magic and aoe attacks that downs you from a relatively far distance.
I had 6 NPC's attacking, but they got downed way too easy, because of those attacks and the regen.

Subdueing them is sometimes the only answer to beat them, but by then its only because the boss itself is unable to be beaten the normal way.