Trismagia serves as the boss for both Dante in Mission 16 and Lucia in Mission 12. This fight is completely scripted as Dante, but for Lucia this is the hardest fight in the game. Because, she have to fight it normally and hope for a good one cycle. Therefore, this is one of the only RNG heavy fights that she had to deal with.
※ The face of the flame is weak to Frost heart, the face of the ice is weak to Flame Heart. No weakness in the face of lightning and merged. The health of the Ice Block that gets thrown at you is 1
The Falling Icicle has it's own HP value :
NG+ Note: Same strategy as New Game.
If the heads are merged Trismagia takes more damage with FireArms. When heads are split, they take more damage with melee. For Dante's case it doesn't matter because he will simply use Majin to finish the fight.
Lucia has to manage DT gauge effectively to kill the boss in one cycle. Her Aerial Cross isn't as good as Dante's Round Trip against this boss either.
All of the above example is demonstrated in the video above. The hardest part is getting behind Trismagia head to deal Aerial Cross damage. You can check the head with Sky High into Spinner Top to see if you are able to Aerial Cross the head or not.
The best RNG head is Ice because Lucia won't get knocked down from it's attack compared to Fire and Electric. Frost Heart is used here because of slow down effect that gives you more opportunity to do extra Aerial Cross.
It's good to check the Frequently Asked Questions (FAQ) if you have general questions regarding the bosses.