Well, there is a difference between a bulky Extremekiller Arceus and a Wallceus. A bulky EKiller will run maximum attack (252 attack + adamant nature), either 16 or 20 speed EV's, and puts the rest in health. The bulk allows it to take hits well, making Leftovers unnecessary; most bulky EKiller run Silk Scarf though some elect to run Lum Berry. Apart from the EV spread, a bulky EKiller functions the same as a fast one. It loses out on the ability to potentially beat Focus Blast Ghostceus (unless it gets really lucky), but can set up a Swords Dance on more threats.
Wallceus, on the other hand, will never invest in Attack or Special Attack, and I find speed creeping it to beat out bulky EKiller to be unnecessary. Physically defense Wallceus run 252 HP/252 Defense/4 Special Defense, whereas Specially defensive Wallceus run 252 HP/4 Defense/252 Special Defense. We already have Terrakion for EKiller and it can OHKO specially defense Wallceus after SR and Spikes. A physically defensive Wallceus with Thunder Wave or Will-O-Wisp, however, will drive Terrakion nuts if Wallceus is at 75+% health. As others have mentioned, Wallceus is really rare so I don't recommend adding a pokemon to specifically dispose of it, but it is something that could potentially bother this team. Darkrai puts it to sleep and can switch in on a Recover or perhaps a Will-O-Wisp (Return and Thunder Wave are dangerous to Darkrai though). Taunt Sableye also handles Wallceus if we elect to use that as our spinblocker. I know Melee Mewtwo, Haruno, and other good players are experienced with using it.
