Two red monsters block the path behind you as you walk into the room.  You can’t leave until you’ve faced the boss.

The boss likes to have a good look around before deciding where to attack.  Wherever it chooses, you want to be somewhere else because the boss’s attack destroys everything on that side of the room.  If the boss’s first attack doesn’t kill you then another red monster will pop-up to help it.

You can shoot the boss if you like.  You won’t hurt it, but you will attract its attention so don’t stick around.

While your shots won’t hurt the boss, it is vulnerable to shots from other red monsters.  So, when the little red monster appears next to the boss you need to shoot it while it is pointing in the boss’s direction.  (Which side you approach it from makes a difference to how easy this is.)  It only takes one hit to kill the boss, and with it gone the other monsters will make themselves scarce.

Back near the entrance there’s a switch on the floor where one of the red monsters used to be.  Step on it to reveal the treasure.