The final boss of Stone Vigil can be hard - I had real trouble seeing the stretches of the arena that would glow before the boss did his strafing breath attacks, and often by the time I realised I was in one, I couldn't get out fast enough... I think it was luck and practice that finally got me through there - learning the patterns in a way you can't from just having someone tell you about them.
One thing that I've discovered since, because I was having trouble with a similar sort of attack that I couldn't see quick enough in Ifrit HM, is that if you zoom your camera out all the way, and can see almost the entire arena, it's a lot easier to see those attacks and get out the way!
This might not solve your problems, since you're getting lag issues, but it's worth a try