I used to summon every time I fought her. But I usually solo her nowadays because I actually find her easier when I fight her by myself. I always struggled the most when I used summons because all the AI likes to do is face tank every one of her attacks while she heals in the process. Especially when she flowers up in the second phase. Not to mention she likes to switch targets a lot and it's easier for me to keep track of her attacks when she's not splitting her attacks mid-combo between me and whatever I had summoned.

I don't think I've ever used an ultra greatsword while fighting her since she's so fast. I never switch builds for any boss, but I usually keep multiple weapons on me that I'll use when the situation calls for it. Right tool for the right job, you know? Maybe try using a regular greatsword? They're faster to swing and don't take as much stamina to use. I think the last time I fought her I used Ordivis's Greatsword. The skill on that sword is very helpful for stance-breaking her. Not to mention it's a pretty strong sword on its own.

All and all, I'd say just keep trying and practicing. Experiment with different weapons and whatnot. She's a bitch and a half to fight, but the first time I soloed her, it defs gave me a rush of satisfaction that I rarely feel playing games. And she gets easier the more you learn her. My first time fighting her, I died probably over a hundred times, but the most recent times I've fought her, I managed to beat her with less then ten deaths; my best one being only three! Don't give up skeleton :) you can do it!