Quote from Flonne;145087:
I don't think Flame Burst gets that good even at r1 on HM missions. Only good for aoe, mob control and stun. Wait till g12 and chain cylinder comes out and it would have more DPS
It's all in the crits. With r1 crit you only need two crits to take out a fighter, pretty much the beefiest enemy you'll run into. It ignores prot so this will happen a decent amount of the time. Even if it doesn't, you can indefinitely spam flamer or throw a windblast in if they get uncomfortably close.
It's definitely very viable on hard mode with high rank crit, and by the time they actually can do hm lvl missions they'd have ample opportunity to rank crit up.
Golem on the other hand, while powerful, is not as fun and rosy as people make it out to be. Making lots of golem crystals is not a fun task. If it's your main source of damage then you probably can't spam Stones adv/HM to get the crystals. Even then, it would take several runs of stones to prep you for just one other mission.
It's definitely a skill you want to rank, but I wouldn't suggest ranking it before you have a solid attacking skill.