Metal Gear Solid Delta: Snake Eater Bosses: Volgin

Non lethal method
Mosin Nagant takes 3 shots per phase to take Volgin down.

Lethal method

by Metsa

Note: with this method, you do have enough time to shoot both the Kerotan and GAKO at the end of the fight during Volgin's death animation

by SuaveBeardson

Mosin Nagant takes a lot of ammo, but is quite fast if applied right.
Remember there's an ammo box right at the start of the hangar when you box teleport in.

Most Optimized Non-Lethal Volgin 1 by Tromboncino

Not much to say here except:

Get good with the aiming and don't shoot too fast with the Mosin because of the spread
Don't be to close to him by the end of the roll animations or he'll counterattack
On phase 2, after you deal the first 3 hits, let him recover before rolling or else he'll drop on the ground
On the very last roll on phase 2 try to not end up too close to the wall, or the camera could get stuck on your big shoulders and you won't be able to see Volgin at all

NOTES:

Not shown in the video because it didn't happen, but the mask also helps in phase one if you mess up the last shots before the transition
And depending on your second roll timing in phase 1 he could start either charging an attack facing his back at you or could just block, which doesn't matter anyway as you can still hit him regardless