Assuming you're talking about Daji. Honestly I had a TON of trouble with Daji also (as seen by my ranting in this very thread... I still think phase 3 is just poorly designed but eh, that's neither here nor there). Make sure every character is at 100 affection since that's the requirement for the true end, and you'll need the magic points you get from it. You'll need to spec your magic so that you have Hell Break, Megito for your main source of magic damage (i think thats what the maxed out fire spell was called?), The second level lightning spell at least if you have the points for it, and the Million Dollar gun. Next use the recollection cubes in the pink bed to fight Llampras get a Llampras tear if you don't already have one, and use the persuade command when fighting Mother Rabbit to farm money. Grind out cash, buy every healing item full stock. For the actual fight, deal with phase 1 normally, then use a Llampras tear immediately during phase 2. It'll make you immune to the gas pretty much making the fight so much easier. Now keep track of your damage and about how many turns it takes to kill. You'll want to get her as close to dead as possible, and then load up your Million Dollar with a full clip. Top off your VIT and WILL, then Cast Hell Break followed by the killing blow to enter phase 3. It's important to have Hell Break active BEFORE triggering phase 3 since she acts twice per turn, and has a chance to stun you out of actually casting Hell Break... or worse, stunning you right into an unavoidable insta kill the following turn and leaving you with literally nothing you can do to stop it. Will Hell Break active as the phase begins you don't have to worry about the stun. Next just unload everything into her. For phase 4, lightning is your bread and butter here. Deal with the dancing as best you can, it can be a bit annoying for sure though. Immediately use wind magic if you're ever grabbed since she works just like Echo, and be liberal with your item usage if need be, but hold on to morphines and edge pills if you can (or just toss lightning drums at her if you picked up some).
Once the fight is over, grab the laser cannon and then go back in (after saving if the game lets you). From there just follow the guide step by step and congrats, you've essentially beaten the base game. Just make sure to restock on ammo since there is one more fight, but its more of a plot fight than anything (unless you're out of ammo like my dumb ass going into it and swinging at it with the starting sword for.... 5 damage....). Now all you have to worry about dealing with is the goddamn Harpy Queen in EX1 oh dear god please make the PTSD from that fight stop, and my naive ass thought Daji phase 3 was unwinnable bullshit when I first stumbled onto it woefully unprepared......
Side note, if you're missing any items or weapons, go back to chapter select and pick them up from the chapter its in. It's been freaking ages since I did the Daji fight but that's basically how I managed it. As someone mentioned before, if your stats are iffy then pick up a charred salamander, use it and reset. High speed is very helpful here, and high attack and health. Throw a few points into magic for sure but no need to max it. Hope this all helps some.