[PvK]

Depends a little on which Ermor and what other magic you have, though I don't have much MP Ermor experience.

If you have Astral magic, then anti-magic and luck items are good cheap choices.

For undead combattants, armor that would normally tire out a living unit can be good choices. Again that tends to be non-death magic. But developing some ability to use non-death magic is probably very good advice anyway. You've already got many good things to use death gems on.

PvK