The strat isn't bad, I would like to add a few notes to keep in mind.
First of all, the Sluagh start respawning at 4 kills it's just a guaranteed respawn at 7 kills, so if you want to make absolutely sure that it doesn't respawn only kill 3 of them (There is only 3 skeletons *hint hint*)
Secondly the skeletons and zombies may be immune to Larelle's necrotic spells, but it will still debuff their attack and they both use physical attacks, plus you won't have to worry about killing them. Plus the Sluagh is weak to necrotic