I just had a party yesterday in Kuftal fighting Ladons, Cocks, and Tigers. If I remember right, the party started out as NIN, RDM, WHM, BLM, MNK, WAR. The melee were taking pretty harsh damage from the Ladons. The WAR disconnected so we fought Tigers and Cocks for about 30-45 minutes waiting for him. We finally decide to get a 6th so I invite a DRG. He used Dragon Killer when we fought Ladons and amazingly enough, the melee got much much less damage than before.
Before the DRG vs. Ladons : I was curing NIN, MNK, and WAR an average of 3500 HP per fight all together.
With the DRG vs. Ladons : I was curing NIN, MNK, and DRG an average of 500 HP per fight all together.
[Taken off of parser and rounded]
I'm not sure exactly what it does but I know it helps a ton.