Been romping with my Undead until...

[Taqwus]

Undead archers would probably be pretty decent against those nigh-unarmored Mictlan troops, and yes the longdead horsemen are nice too (and Pale Riders is definitely a bulk summons... along with Carrion Reanimation and the nice but hard-to-research Wight summons.)
Disease might also be a nasty weapon against Mictlan; think black servants with bane venom charms, harbingers, black death, etc. Mictlan *needs* blood magic, which effectively means they need to have a decent population in at least some provinces, plus they need to keep unrest down. Hitting their high-pop provinces with pop killers (which, incidentally, should also increase unrest) should help.