A good lever to have in the game for if summons ever get out of line in their capability of solo tanking bosses would be the bosses having a passive that multiplies damage against non player characters.
If there's ever a mercenary system it would also fall into this and if there ever is a "protect the NPC" quest the passive could have a negative value if needed (if for some reason you didn't want to specifically buff the NPC, say in the corner case that you want the normal aggressive mobs to do full damage but nerf specifically the bosses damage to the NPC or if you want adversary players to be able to do full damage to the NPC and there's a boss in the same area).