You can't kill Balrahn, iirc. He'll sit there with 1% HP and start taking no dmg. But that makes it very hard to get the TP to use aftermath 1,2, then 3. I also seem to recall getting some regain out of nowhere during this.
Scared the ***outta me when I was making my first mythic(Ryu.) Cause I was killing him too fast and thought I might kill him or time out due to no TP gain. Was terrified I'd have to get my trophies again...
Keep in mind, you need to wait till he uses the mythic WS before you start going through the various aftermaths/
/SAM and sekkanoki are very useful for this fight. You can build up 3k TP while waiting for him to use the mythic ws, then turn away. Once he WS's, you can sekka, WS, WS to do AM1 and 2 rapidly, and then meditate to build towards AM3.