Okay, I am at the point where I barely die to the big L. I think having one helper is better than two.
First Phase- Try to stay on both sides of him and make him choose one of you. When he does the other can attack his back/legs for a few hits and sometimes a nice combo. When he turns, the other player can do the same. Just watch out for his big slam/aoe attack.
Second phase- do the same but bait his forward fire spew. The player behind him can stay pretty close and he takes a while to start the attack, it is really telegraphed. The player behind can move in for a nice combo and roll out, while the other player can hit him from the other side. Repeat until dead.
Just remember to never get caught in a corner and I found not locking on was so much easier since you can easily roll away from all of his attacks. Fire resistant gear seems to help too.
Solo is a different story
I have been co-oping him all morning and this has worked almost all of the time. Good Luck, GAF.