Heroic modes add two additonal mechanics to deal with: Iron Tomb which is a simple "move away" mechanic and Iron Prison, which deals 100% of your maximum life when the debuff expires. Iron Tomb increases the movement required on the fight, which benefits FDCL. Iron Prison is not mitigated by armor, so just disperse when it's falling or call for externals if you are on cd.
With the extra movement requirement on heroic, FDCL gains some ground on SWI. I've found SWI to provide better results in general, but the spread is relatively small if you prefer FDCL.
ToF uptime is high on the fight, so it's the most dps. Mobility from DI is helpful if you prefer it, but don't take it and FDCL together.
Halo can be a pain to position for, but it's the best dps choice. Synch it with Foustrem Totem adds for maximum output. Cascade isn't worthwhile and the healing from DS isn't exceptionaly useful since the raid won't be stacked.