Zeromus is the second esper in Stilshrine of Miriam. To get to him you need the Stone of Condamner (look in "Silshrine of Miriam" part, at the end).
+ Dark Lords
About 170 000 HP
Resistant to Libra,
Expose works on him
In that battle you cannot use magic, so remember to buff your characters before. Change their healing gambits to objects. Set it like that "Ally: HP<60% ---> X-potion". It is important to have all three Potion Lores and at least one Pheasant Netsuke (the more, the better). X-potion with three Potion Lores and Pheasant Netsuke restores 3600 HP. You should give Hermes Shoes to your strongest hero, to let him have Haste all the time. It's also important when one of your characters has his gambit set like this: "Foe: HP<5000 ---> Attack", to take care of Dark Lords, that appear all the time. Others should have "Foe: HP>=5000 ---> Attack", to care only about Zeromus. I don't have to mention that big reserve of X-potions and Phoenix Downs is necessary, so as Haste (or Hastega) Mote is.
Attacks of this esper are based on graviga. His normal Graviga consumes 25% of max HP level, and Piercing Graviga consumes 50%. Two attacks and whole team will be dead.
It is worth to use Expose at on Zeromus at the beginning since magic is not allowed.