[Taqwus]

Luck charms, barkskin amulets aren't too expensive and can help (well, the barkskin won't help as much as on a human assuming that Empoisoners, as lizardmen, have some base prot; can't remember what it is 'tho). For somebody with decent prot (perhaps from Boots of Stone, say), the etherealness from a Robe of Shadows can be a big help.

If you think there's a decent chance the assassin will die, a weapon with powerful poison or a decay effect might be in order.

But the auto summoners are very useful (wraith crown in particular; but that's very expensive; against many non-flying non-lightning-immune opponents, a spirit helm might be the better buy, although harder for C'tis).

Later on, the Drain Life banner is just vicious... always-hits, affects even lifeless enemies, decent range, good armor-negating hp/fatigue damage, restores health/fatigue on assassin, and not blocked by any immunities.