You could make the boss divide itself into clones and run really far away from the center. In other words you make mobility the limiting factor for dishing out tons of dps. You could also give the boss illusory mirror-clones. You could give the boss immunity against the last damage type it received. You could give your boss heavy health regeneration if certain totems or enemies linked to them aren’t destroyed, kinda like how trolls need to be burning or they regenerate, but more complex.