Fortunately he can Skill Swap Mummy onto anything, not just physical attackers, and there are plenty of useful abilities to swap for. Regenerator, Magic Guard, Natural Cure, etc. Unlike Trevenant this guy can actually take hits themselves, he just has a harder time recovering HP, but honestly with Rest and a cleric he tends to do fine overall, getting plenty of switch in opportunities. I'm really digging physically bulky WoW+Calm Mind sets currently.
Coff@Leftovers
Mummy
Bold 252 HP / 252 Df / 4 SpD
~ Calm Mind
~ WoW
~ Hex
~ Knock Off/Skill Swap/Substitute
WoW and Knock Off with his Mummy ability limits so many switch ins for your opponent. Virtually nothing can disregard losing its item AND getting burned. This allows opportunities to Calm Mind and patch up your SpD and power up Hex to your burn victims, becoming really powerful. The only common sight that is immune is Blissey, but even Blissey can be stalled out of Softboiled with CM boosting your SpD against a paltry Flamethrower, being immune to Seismic Toss, and Knock Off removing her Leftovers. The biggest problem is Toxic and strong special attacks before you can boost enough, or Fire pokemon you can't burn. But a Wish passer and Aromatherapy user allows for numerous future switch ins. Even if he doesn't end up killing anything, the items you removed and burns you caused can often lead to your victory in the end.