Freezing cloud is basically useless against undead.
Poisonous cloud is basically useless against undead.
Conjure flame is basically awesome sauce against undead, and combines very nicely with force lance for monsters too smart to initially move onto flaming terrain.
Lightning bolt also kicks undead ass, but in a very hey, look at me, I am an attention whore sort of way.
Actually, beyond Lair and the Lair runes, the set of poison resistant monsters who are NOT also cold resistant is pretty tiny, and I will still have bolt of cold.
Plus, clouds may dissipate faster out of LoS, but already applied poison doesn´t - its the gift that keeps on giving.
Having said that, I am still reluctant to forget Freezing Cloud, because its basically my favourite level 6 attack spell.
Getting access to Ignite Poison would cost me 1500 gold and another 5 spell slots, hmm. Probably not worth it.
EDIT:
So I forgot FC, memorised CBlink, and went to Vaults 5, basically did what I said I would do: PShift, Haste, Crystal Ball, Down, CBlink, PCloud, PCloud, run, CBlink, run, rest. PCloud, PCloud, Crystal Ball, Bolt of Cold, Bolt of Cold, rest. Run to stairs via unused corridor, back up to V4 to chill. Stair danced a little bit (much less stuff in the danger zone), and currently the stair area is completely clear, so as an approach, this might work.
I had totally forgotten just how awesome CBlink is. I mean, I have semi controlled blink/ firestormed the Vaults welcoming part in the past, and it was good, but this is like, holy shit, if I die here I have only myself to blame.
I am going to bed now, to tackle V5 again another day.
Also: in combo with my nightstalker mut, Ash helped out massively - avoiding red } in favour of brown and white ones really made my crazy dashes past the corners a lot safer.