MCH Heat Blast

[Kalise]

I mean, do I just spam it during the 8 seconds of HYPERCHARGE and use GAUSS ROUND somewhere during HYPERCHARGE?

Yes.

During Hypercharge, you'll want to be spamming Heat Blast and Gauss Round (As Heat Blast also recharges Gauss Round)

After level 50, you'll also be spamming Ricochet too as it is another oGCD skill that is recharged by Heat Blast.

A point to note, while it does not seem very strong with only a 220 potency compared to your normal combo's 180/260/340 potency, one of the things is that it takes your GCD to 1.5 seconds instead of the normal 2.5 seconds allowing you to do ~60% more attacks in the same time frame (In addition to allowing you to use more Gauss Rounds/Ricochets)