I don't know if i understand how to replicate the glitch but i could try making some runs with it, i will be back here to tell if i get any better results. congrats on the find, this is game changing.

Second phase Move your gunslingers to the space in between your initial position and boss spawn point, after that, watch out for boss attacks with counters and nuke it before it attacks a lot, the more it attacks, the more time you lose. if he uses the skill to mark a unit, just explode and nuke it with everything you got.