Hard Mode Pyromancer on Ultimate Viessa -TFD

In pug matches I'll average 53mil dmg but the highest i've seen was 69mil dmg for a hard group carry. Important note is when running the Polar Night set you need a weapon with Chill Atk to building and maintaining the crit dmg stacks, relying on skills alone is not consistent. Skill priorities are in the order of Cold-Bloodedness, Blizzard, Cold Snap, Frost Shards.

Stats:
DEF: 5,865
SHD: 660
HP: 18,717
MP: 293

Reactor: Frozen Mechanic Reactor with Thundercage Mounting and Bonus Skill Atk vs Colossus/Crit Dmg.

Polar Night Set:
Auxiliary: Max HP / MP Recovery out of Combat
Sensor: Max MP / MP Recovery in Combat
Memory: DEF / MP Recovery Modifier
Processor: Max Shield / Shield Recovery Modifier

Modules: Cold-Bloodedness, Long Distance Maneuvering, Increased HP, HP Amplification, Focus on Chill, MP Conversion, Skill Expansion, Dangerous Ambush, and the four crit Mods.

If you want more tankiness you can swap Emergency Measures for a gold elemental resistance mod and pair it with an elemental resistance stat on one of the external components.

For the external components you can swap out MP Recovery out of Combat, MP Recovery in Combat, MP Recovery Modifier, and Max Shield for elemental resistance stats.