Capped attack speed for players. Cannot be surmounted by potions, speed infusion, boosters, etc.
Capped damage. At a very low value like 50k. Can only be broken by temporarily breaking through its special defenses. This is only done by triggering a special weapon that is part of the room (attack the reactors spread throughout the room)
Teleporting and other skills are disabled or greatly nerfed, however there are teleporters placed across the map.
To ensure that people are actually paying attention instead of just mindlessly attacking, implement that Pierre mechanic where randomly chosen players end up healing the boss if they attack while marked.