I don't personally own the gun but would like to, and have been intrigued by the charge attack and it's possibilities.
Cutter Line+Tortogun: Using the charge to create a solid wall behind targets, followed by chasing enemies into said wall using the cutter line. Damage from both cutter strikes and the ticks of crystals.
The same idea above could be applied to heavy swords by knocking enemies into the shards created. Perhaps even use a nitronome to push enemies into the shards, for effective crowd control.
Vortex line + Tortoguns: Straight forward, you get them in a huddle and then unleash a charge on enemies, shards hitting multiple enemies (perhaps invincitinks reducing effectiveness)
Stagger Storm + Tortogun: Slow enemies in confined spaces so they are hit as much as possible by the shards
Self Protection: Fire a charge to create a partial barrier in the corner of a room and be protected from enemies. Alternatively could be used to trap larger dangerous enemies.
Any interesting tactics people have come up with you should post here!
Calibur + Tortogun:
Like your suggestion on Cutters and Tortoguns, but you use the Calibur charge instead to hit up to thrice if done correctly with setting up the wall. Having CTR Max on the Calibur is probably more important than having it on the Tortogun for this setup.