Warn him.

Your boss might not be particularly nice, but he's still your boss. And hell, maybe he will show some gratitude if you save his ass from a potential sneak attack.

"Look out behind you!" you yell, pointing at the archer. The necromancer rolls his eyes, expecting it to be a cheap trick, but his skeletons turn their skulls towards the direction of your finger. At the sight of the hero, they immediately rush into battle. This makes the necromancer take notice as well, turning around and taking shelter behind his skeleton bodyguards, causing the archer's arrow to miss its intended target.

You take the opportunity to get back up on your feet and run off while the necromancer and his skeletons are momentarily distracted by what will certainly be a short battle, given the archer's lack of teammates and presumably low health. You manage to get out of the boss room and stop to take a breather in the corridor, hoping they can't follow you here. Maybe you could go elsewhere and hope for more successful outcomes than being scolded by your boss?

The answer turns out to be no, as The Keeper suddenly forms from the stone bricks making up the floor, standing before you with halberd in hand. He must be done chasing that hero. You only have time to let out a disappointed groan before he splits you clean in half, erasing you from existence until the next respawn opportunity.