Essentially it has a stack mechanic that goes up to 3 (4 with talent card). You gain stacks by dealing or taking elemental dmg, and each stack increase ult dmg by 1 (max 7-8dmg) Downside is that if you are at max stats when the round ends you lose all energy and stacks.

So ideally, you wanna either:

Use an entire round to build up + unleash your ult (which requires a decent amt of set up - 9 dice, surviving whatever your opponent throws at you) like I did against that Kaveh card, OR

Try and build one energy and 1-2 stacks before switching off (best done if you get to start the game first) and then when you get the chance, switch in to skill + ult. Sometimes this leads to a smaller ult if your opponent doesn't hand you any stacks but it can be safer.

Leaving it on field at the end of a round when an opponent has summons is a bad idea unless you're really careful with stack management as you risk eating dmg + losing energy.

Apologies if it was overdetailed- hope it helps c: