Not encountered that at all. Are you using the float option on the trainer, or not using a trainer at all?
And what patch do you have installed with the game?
It only happens when I run the trainer.I am running with the latest patch and the latest trainer.It has only happened in this boss fight.I don't have the float option running at the time.I can get by though I love this trainer and the float option is sweet,without it I won't be able get through this game.If they ever do a new mega trainer unlimited horse stamina,unlimted ammo and horse superspeed would be the icing on the cake!
I had a save from the Guardian fight, and used the unlimited health and it was fine.
Did you have 1 hit kill on? If so it may have whacked scripted events out of sync.
I had 1 one kill on for a while then took it off,maybe that screwed it up,I got through this without this trainer and so far the problem hasn't returned.Maybe I'm the only one that has had this happen.Thanks for looking into it.