Actually Merciless Judgment Dragonheart doesn't do a set amount of damage. It is like Ultimecia's ability, it just knocks you to near 1 HP. You can never actually die from Merciless Judgment, it will be the poison or other attacks that will end up killing you.

And you will want buffs on you if you want to get through the fight quickly enough to avoid getting Death from the boss.

I used Vanille, Fang and Sazh in my party. Vanille has Curaja and Curasa which will be about all the work you need. Now I played very aggressive against the guy because I had fought him so much that I knew what to deal with. The above stuff outlines a lot of the stuff already.

My setup for Paradigms was this
Medic, Ravager, Commando
Ravager, Synergist, Commando
Ravager, Ravager, Commando
Saboteur, Ravanger, Saboteur
Ravager, Commando, Commando
Medic, Synergist, Commando

For the fight I defaulted to Ravager, Synergist, Commando because Sazh will be allows a few seconds to get in buffs before the hit comes. You can do Medic Synergist, Commando if you're very paranoid. But once Merciless Judgment comes which to Medic, Synergist, Commando and keep it going until all HP is in the Green even if for a moment. Afterward, go back t Ravager, Synergist, Commando. Work on that Chain Meter for stagger while Sazh is finishing up doing a full buff and you want a full buff on all 3 characters.

Once Sazh is done buffing, switch to healing on Vanille when you do need and just keep up the pressure on the enemy with the Chain Meter. If you are aggressive about the Chain Meter you should get him for he goes and used Merciless Judgment, which will reset the Chain Meter among other things. The only thing you really have to worry about from the boss between the first and second merciless judgments is the hand slapping for small damage and the random orb shooting you. You'll want to keep your HP high enough so that thing won't out right kill, it does about 2000 HP damage.

Either way if you get the Chain Meter before the second Merciless Judgment then switch to Saboteur, Ravager, Saboteur and have Fang and Vanille cast until it has deprotect and deshell at least. I've noticed in Stagger debuffs have a higher chance of success, having more Saboteur in your Paradigm also improves because they help each other. Once you have those two debuff switch out to Ravager, Ravager, Commando until the stagger is up around 800-900%. Then switch to Ravager, Commando, Commando. You can do it earlier, but more than like Fang isn't strong enough to get capped out until you start hitting the 900s.

Do note that the boss always uses Merciless Judgment once it recovers from Stagger so be prepared and switch back to Medic after the stagger is over or almost over.

Now the boss works in cycles. The first round is pretty easy because you're just dealing with the hand slap and random orb shooting. The second round the boss lands you in with a lot of status ailments. It is important that you have Mallets and Foul Liquids on hand for you at this point. They are cheap so just buy like 30 each. Mallets remove the magic off and Foul Liquid removes the sleep like effect. If anyone gets the magic off use a mallet, it is more important than the sleep.

The reason you fully buffed is because of the status ailments that the end will throw at you. The buffs you have can sometimes cancel or prevent the debuffs he will throw at you. They will sometimes just cancel each other out, which is far better than taking it. If you do take anyone make sure that you remove the deprotect and deshell at the very least. The best way to do this is just pull out your Synergist and Sazh will override them all with his new buffs. It will be a battle of buffs and debuffs and it is quite likely you won't get time to get the stagger meter up, but if you always keep a Ravager in play when you're not healing you should be able to get him back into Stagger. The important thing will be to go for Stagger as quickly as possible on this form so that it will change back to the first round, which it cycles to after the next Merciless Judgment. If you get poison remove it with an antidote from everyone immediately. If you have poison on you when a Merciless Judgment goes off you will die.

When the enemy's HP reaches a certain point it will automatically do a Merciless Judgment, usually this happens during your staggered effect. So if it happens during then you know you've made it past the worst part. Now always make sure your HP near to the green and keep up the same pressure. The status ailment attack will all stop. So all you have to worry about it AoE spells and Death. Keep things going and you should be able to kill him.