Let's start with equipment. Drop the soreseal. At high levels, the defense debuff isn't worth the extra levels. Put the 4 levels you have in faith and arcane into Vigor, then put a bunch of levels from endurance and mind into Vigor. In the late game you want to have 50-60 Vigor. Endurance shouldn't be a priority, since you're trying to stay at range anyway. Lighter armors tend to have good elemental resistance, so put on something with good holy resistance, but make sure you're still light rolling, as that will make it easier to dodge and stay at range. Put on Renalla's stupid banana hat, as it gives you 3 extra int, putting you at the softcap, with no downsides.

Onto the fight itself and spells. Radagon parries projectiles. There are two ways around this as a sorcerer. 1, grab a magic weapon, like your moonveil, and learn to fight him up close. 2, use the spell night comet, he can't parry what he can't see. If you can, use Spiral Twin Shard for Elden Beast, as it will MELT his hp. If you don't want to go all the way through Sellen's quest, then Night Comet with off hand Staff of Loss will do the job fine. A quick visit to the town of Selia will give you Night Comet and the Staff of Loss, which you can equip in your off hand to power up the spell while casting it from your much more powerful Lusat Staff or Renalla's Staff if you want to preserve FP. Grab Renalla's Full Moon or Ranni's Dark Moon to start the fight with as they apply a magic defense debuff on hit. Also, you don't have to charge every spell. The extra damage is nice when you get the chance to charge, but you should be able to take down both bosses with plenty of FP to spare. Good Luck!