He who fights and runs away, will live to fight and run away another day! 
It wasn’t even sarcasm bro. I was serious lmao. You cannot stand still with him at all.
lol well thanks. It’s hard to tell on the internet sometimes.
Why didn’t i think of this earlier!!! Get an Antagonist shield, and exploit the reflecting bullets. Stand really close, and shoot! The bullets will reflect off BA Rex, and with a good Antagonist shield, they will bounce off of you as well. This turns into a loop, and since the bullets that are deflected off of you will increase their damage by 800 %, it will build up very quickly.
The deflection chance is… what… 50 %, so you’ll likely die a few times, but this works on any character.
But you’re playing as Maya, right? Taking out one of the barrels should activate Kinetic Reflection. If it’s active when you go for the kill shot, you will not take any damage at all if you equip a Trickster COM that gives you at least 10/5 in Kinetic Reflection. But the Antagonist is the key. Without it, you won’t be able to build up the dps.
Come to think of it… This should work on some other tough enemies as well. I know it works on Hyperius, but Jackenstein… Might be worth a try. 
Oh. cool!