So after my initial boss success with margit and godrick I fucked around for a bit in limgrave and liurnia and then finally headed to raya lucaria, cleared that put okay, and then ran into an absolute wall with Rennala.
Magic vs magic was probably always going to be a tough fight but dang she was brutal. Over a few days (playing an hour or two each day) I got her first phase down, but couldn't figure out her second. I got super close a number of times (the most painful: dying as her health bar ran out and her final line of dialogue hit).
It was an interesting play combination - on the one hand, repeatedly testing myself against her really improved my skills. I got noticeably better at reading things and knowing how to react. On the other hand, in spite of my best efforts to keep my patience, I was getting very frustrated. So I left her alone, went and got a few more levels, switched the sword I was using, and beat a couple bosses. And then I went back and beat her on the 3rd try.
Also, I realised pretty early on that I needed sword for phase 1, because locking on to cast when there's a billion kiddos in the room was a nightmare. But I ended up using a sword for both phases to beat her. The animation time on my casts (especially rock sling) was way too long, and she fires off so many spells you've got to be able to swap attack and defense as quickly as possible.
It's very nice to have overcome that hurdle. I'm excited to try some of the other tough bosses now. Not sure exactly where I'll head next. The game is pushing me gently towards the atlas plateau, but I might fuck around caelid instead, go see how I manage radahn. I want to make sure I'm getting my sidequests in too, I've got ranni's quest up and running, and nephali's, but I still need to get fia's going properly.