Tooltip is bugged in Definitive Edition, at least for me. Does about 3x as much damage as the tooltip suggests when hovering over it on my Mage.

There's a bit of cheese you can pull off with this skill. If the rest of your team is in combat but the caster is not, casting this spell on an enemy will not bring the caster into combat. That means you can freely go ahead and plop Flaming Crescendoes on every enemy before joining the battle for some huge initial damage in the first round. Though keep in mind that enemies will move so you might blow up your own party too.

There is a staff with this skill you can buy or steal from a lizard in Kemm's manor upstairs. You use Onslaught with it to stack 5x flaming crescendo per hit for some hilarious OP'ed moments. God I love getting creative in this game.

Good combo with bloated corpse. You can Flaming Crescendo the Corpse, Tele it or just let it walk to your enemies and explode to deal massive mixed damage.