Heya, you just have to learn there routeen, >_> its been some time sence ive played the game, but Il give you a few tips...

First off, I dont recomend gun slinger for this fight. Trickster or Sword master are one;s I prefer >_>....Never actually found gun slinger helpfull in a boss battle.

Next, learn to love the Roll butten, when fighting them, its good to keep your eye on both of them.

When they both run away to do that jump moove, Let them, and be sure BOTH fall down before you attempt to attack them!, or one will squish you >_>;....

Also, Deal equil damage to both of them!, if you kill one, the other gets harder, so if you lower both of there health really low, the other wont be as hard to kill.

hope this helps! 

Alright, Well, What is your problem than,

Are you just getting struck too mutch?, I usually just tsit there and pound one , and when I see the other one going to attack, I jump up, and then perform a helm breaker. >.> So lets focus on whats getting you killed, and see what you could improove on.

Maybe this will help:

7540 from GamesFAQ:

-------AGNI & RUDRA------- DIFFICULTY: VERY HARD

ATTACKS: FIRST FIGHT
1. They're first attack is a slash and dash method they use very often. You
know when this is going to happen when they return to there spots, then say
something like "Leave now, mortal!" which follows up with a dash then a slash.

2. They're second attacks is the normal sword play with a follow up of deadly,
continuous slashes. This is a freaking hard thing to dodge or even get out of
once you get caught in it.

3. They're last attacks are a jump in the air and slash down to the ground. You
can fortell this pretty easily, but won't have enough time to dodge.

ATTACKS: SECOND FIGHT

1. All of the attacks are pretty much the same, except that there is two swords
instead of one, so prepare to get your ass handed to you.

DEFEATING THEM

They both have different health bars and is a pain to do damage. Prepare your
self with Vital Stars and a Blue Orb. You're definitly gonna need it. Equip
yourself with Cerberus too, since that's Agni's main weakness. The Rebellion
works fairly well, but doesn't quite catch up. The first thing to do is slash
at them before they attack which is common knowledge to all DMC fans. Always
be fixed on both of them since there is a surprise at the end. When you've
wasted one, the other will go back to his guard and call upon his brothers
sword for double inferno. Prepare to get kicked like a bucket.

----ALTERNATE STRATEGY------PREVENT THE TWO FROM FUSING-------from Tyron G.

You must stand guard and make sure that he gets nowhere near his guard tower.
This means laying a flipping hard beating on the remaining brother. Use the
Cerberus to lay more damage.

DEFEATING THE FUSION FORM

This is the toughest part of the battle. He blocks almost the whole way and
if you flunk, you die. You should reconsider changing to Royal Guard for this
one. Trickster will get you nowhere and jumping around isn't the best option,
Swordmaster is perfect, but think about it, will you be using your sword when he
keeps on blocking it? Gunslinger is also effective, but doesn't do any serious
damage. Keep guard, pull back then attack. Keep that phase in mind.

J-Cena.com