Wait, why can't you use the guardian weapons on the glaive? From what I've read, they work just fine, they just get slowly damaged over the course of the fight.
Okay, went back and reread what you said, voice dictating so it's kind of hard to edit, what I meant to suggest is, when your Shard cannons starts becoming damaged, and malfunctioning, you would start repairing one of them then. That way, it would be fully repaired and ready to go again by the time the others are getting more damaged, you start repairing a second one, keep fighting, and then when the third one is completely damaged you repair it. You should be able to keep two of your three Shard cannons functional at all times.
I am a little bit confused as to why you are using a single multi cannon, though. They are best at killing hearts, and Shard cannons are also extremely good at killing hearts.