So I was having really persistent issues with bells ruining my solo flawless runs on the final fight as well, and today I finally got it done. The biggest change I made was to no shoot the middle bell at all on the Citial fight, I kept dying doing this but never died once shooting the left or right Bells only her the boss room. The first two fights were totally fine, I teleported back and forth several times on Ghalran and on the vault encounter and it worked each time.