TASOGARE YORI MO KURAKI MONO
CHI NO NAGARE YORI AKAKI MONO
Her explosion is more than powerful enough, but I doubt your average servant would let her use it, instant cast or not.
she's like a tactical missile you can use once a day if you can make up for her lack of mobility
besides fate has built-in ways to restore mana, so she might be able to cast two explosions with your help
Deermud had B magic resistance and both are too fast. It has to be someone slower moving in a predictable path with enough time to charge an explosion.
Iskander using IH sounds like the perfect target.