Some builds are good for bosses others not. So let us focus about what we are good.

Create some special expedition just filled with bosses. Example 4-12 bosses each expedition.

For that expedition you could give nice rewards. Example potions tier from 1 to 20 with passive effects (always enabled). According more player kill harder and high bosses tiers they evolve or drop better potions tiers with better status/effect/skills buffs etc.