Very nicely.

After perhaps a few hours of standing around with the other monsters, which consist of three orcs, two skeletons and another dark caster, you hear footsteps and talking. You count four voices, and prepare for battle. That is to say, you run off and hide nearby, attracting some puzzled looks from the other monsters.

You peer out from behind a corner at the area before the boss door, and watch as a part of four heroes clash with the monsters. You're too far away to do any casting and you don't want to risk showing yourself, so you're content with observing. The party consists of a barbarian, a warrior in heavy armor, a mage and a healer. They easily dispose of your comrades, and nonchalantly enter through the boss door. You've never understood how heroes can be so casual about what they do. Even when entering a necromancer's inner chamber they treat it like it's not a big deal. They must be doing stuff like this regularly. You find it hard to imagine such a lifestyle, but then you find it hard to imagine a life outside the confined of your dungeon as well. It's a literal dungeon to you.

Bad puns aside, you sneak up to the door, finding that it's closed behind the heroes so that they cannot leave until the battle is over. Apparently more heroes are allowed to join the battle though, since the door opens as easily from the outside as last time. "Easily" meaning some more dumb-looking attempts to reach the door knob.

You open the doors slightly and peek inside the room, watching the ongoing battle. The skeletons are engaged in combat with the heroes while the necromancer stays behind them, empowering them and directing them with some form of aggro-control spells, from what you can tell. Now you just need to wait for the moment to strike...

You stay where you are until you can see the necromancer's strength waning. He looks hurt and he uses his magic more sparingly to save his mana for reanimating his constantly dying skeletons. You decide now's the time and wait for him, the skeletons and heroes to be looking away from you, and as soon as all eyes are turned away from the door you slip inside and quickly but quietly rush close enough to the boss to cast a Hex on him, and then you rush back out just as fast. However you can hear behind you that he noticed your spell, and the heroes have enough awareness of your surroundings to see you as well. Fortunately the heroes stick to their main target, which forces him to stay focused on them.

You run as far away from the boss area as you can, not wanting to be around for if the heroes come back out to chase you. Once you think you're safe, you stop and lean against a wall. You take a moment for your breathing to slow down, and then smile once you feel a sudden burst of XP inside you. The boss just died, and your Hex was still in effect. Your strategy might work after all...