If you can squeeze in Flaming Crush instead of magic bursting, without breaking skillchains, it tends to do ridiculous damage on older content like Vagary. Especially with Frailty+Malaise.
Ideally the SCH would do Fragmentation, so you can extend the skillchain with Flaming Crush rather than bursting with Thunderstorm. If Crush damage sucks, you can just revert to bursting with Ramuh, or they can use fire and you can burst Conflag/Meteor Strike.
That said, it's different for every zone and boss. Flaming Crush is best on 80% of the bosses probably, and for trash mobs in the fomor zone. If you want advice for a specific zone feel free to ask.