Re: Fulminant Prism:
Fulminant prism can be challenging to use correctly, but in general you can predict the enemy's movement if a) they are not batty and b) you understand how
Bresenham's line algorithm works. I find that if I have a loose cluster of enemies, relatively open surrounds, and a clear line of retreat, I can heavily damage most enemies in the cluster via cast+retreat.. (and then perhaps pick them off in sets of 2-3 using DSpray).
It's actually one of the spells I would prefer not to learn, due to how much attention you need to pay to do just one cast, but it's undoubtedly useful.
I would also highlight the 'blocking a hallway temporarily' use that damiac mentions -- since this can cause enemies to choose a different path to you, it can be a decent escape tool.
If I found an appropriate book, I would probably go for Mephitic Cloud instead. Though my experience is that it's uncommon to get such a book at the time you would want it.
+1 on the use with statues. Statues are often pretty scary, so attacking them from just outside LOS is a very helpful thing to be able to do. That said, you often don't encounter very many statues in a game.
Iskenderun's Mystic Blast is good because in addition to dealing respectable damage to the initial target, it can explode and damage other adjacent enemies (by the same amount? certainly a significant amount).. You can hit up to 5 enemies at once. (that's what I notice about the current spell set you are using -- they both hit a single target each turn. Isolating individual targets is not always practically achievable though.)