[Aerich]

Stunning certainly does work on undead. You can't Hold them, except with a specialized Hold Undead spell, but you can stun them with Chromatic Orb.

Especially in areas where there are lots of weapon-immune undead, I like to Web up an area and cast multiple Spike Growths. Typically you can get through two such areas before having to rest again, if you save most of your single-target stunners for a second area.

There are some spells I use a lot more on HoF mode than on normal mode. Hold/Charm, Curse/Prayer/Recitation, Spiritual Hammer, etc.

Spiritual Hammer in particular is great. It's essentially a magical ranged weapon, which you can really use up until you can buy +2 sling stones in the Severed Hand.