Mumbo's probably the weakest one here and has the most obvious weakness. The moment someone takes his hat he's fucked. He could probably try to evade and dodge, but not with 8 opponents with potentially greater powers.
Is Magic Man even a reality warper or just a powerful spell-caster? The greatest thing he did (off-screen) was turning Mars's ground into cheese or something IIRC. Either way, he acts like a literal stoner and would probably not take the fight seriously at all, getting wrecked eventually.
I don't know if HIM is a reality warper, either. Other than just "being Satan", there's not much to his power. He can get stronger from others' misery I think? He turned super huge in that bad future episode? But that was from an entire town. In his base form he'd probably lose.
Joker has 99% of Mxy's power so he's not the strongest here by default. But I guess Jokerwank could potentially lead to him outsmarting Mxy maybe? But either way, his focus probably wouldn't be Mxy at all, he'd spend most of the fight trying to out-edge Bill.
Bill is kinda retarded, maybe Joker could outsmart him too. But if he stops acting retarded, he could wreck most of the others playfully. He can affect other people's bodies from a distance, but for some reason he didn't do that when chasing the twins. He also has a very obvious weak spot (eye). Someone blinds him, he rubs his eye, at the same time that someone kills you.