Are you using potions? If not -> use potions

The easiest way to fix your situation is to do a little DPS stop, survive the 3rd heavensfall (its not hard) and push him over then. However, your PLD could use HG to nullify the first set of claws (it gives your healers a little break too). Otherwise, you want to optimize your dps. I saw several possibilities which work, the best was (DPS required) having 1 dragon und two heavenfalls. Instant melee lb on add and finished off by the melee himself (with only a little help by bard) and sticking to the boss for the rest with WHM single heal the shit while SCH was dpsing too. The most safest possibility I know (when DPS is a bit slacking): Kill first add normal, Caster LB the second along with boss.