The best run I have so far is with Supay + Aphrodite + Hestia + Poseidon, which has a record of 14 minutes for 20 heats in Olympius.

I got Flame Flourish (skill applies Scorch), Burning Desire (Scorch doesn't expire on Weak foes), Glamour Gain (automatically applies Weak to nearby foes) and Island Getaway (all foes are considered nearby) just in the first map. Later on I also leveled up Pyro Technique (Scorch deals damage faster) several times. Every encounter is almost instant at this point. All I need to do is dash through the whole map and apply Scorch to all foes. The only weakness is that boss fights are pretty slow, as the only source of damage is Scorch. But that's still my fastest run.

The second fastest run I have is with the default Black Coat + Zeus + Hestia + Demeter, which is a 17 minutes run for a 20 heats Olympius.

Again, I got Heaven Flourish (skill applies Blitz), Arc Flash (omega moves immediately activate Blitz) and Thermal Dynamics (Blitz applies Scorch) in the first map. Later on, I got Freezer Burn (applying Freeze immediately deals all the remaining damage from Scorch with a x2 multiplier) in the second map. Encounter clear times are no way close to my best run above, but the boss fight is pretty crazy. Two omega skills plus one basic to trigger Freezer Burn can easily deal over 5K.

Do you have any fast or fun builds you'd like to share?