This should work.
This will most likely fail.
Still best overall plan I have seen so shall go with it.
[X] ATTACK!
-[X] Conjure Spear x5
-[X] Conjure Chain
-[X] Enchant them: Flaming, homing.
-[X] Conjure the spears around the monster and throw them at the monster´s Head, left arm and right arm with each spear having a chain tied to it.
[X] With the last remaining spear rush towards his gigantic arm and hammer it down his elbow while enchanting it with a [Explosive] property, fill it with magic and set it to explode right in his elbow, evaporating his arm.
-[X] Use chain barrier to protect yourself and nanoha from the explosion, as soon as you´re able grab her and retreat to a safe place, cleanse as soon as possible while making sure she´s okay.