Hex him.

Your boss appears to be a total scumbag. So you decide to side with the archer by helping him out, casting a Hex on your boss. You're somewhat surprised to find that it actually affects him, and he does too.

"You dare use your amateur magic against me? You pathetic worm..."

The archer then releases his arrow, and it pierces the necromancer's back. He lets out a brief yell before bursting into black smoke, and the skeletons fall to pieces around you.

Your mouth hangs open. Not because you just assisted in the **** of your own boss, or because he was actually low enough on health for your trick to work, or anything like that. No, it's because you actually felt it. The XP.

You got XP for your boss being killed while under the effect of your spell. And a lot of it. Not enough to level up, but almost. Nevertheless, for the first time you actually got enough to feel that you do indeed get some XP from an assisted kill.

The moment of shock and bliss is interrupted as you see the archer ready another arrow, aiming at you. Ah right, you're still a monster and he's still a hero. He probably didn't even notice you helping him. This is... not ideal.