Just keep shooting him use explosives they are the quickest way to kill him if you use all the rockets on the rocket launcher ( the 1st you unlock cant remember the name
) it takes a lot of life i almost killed him on solo using that gun but he killed me lol. You can tell how much life he has left by looking at his light, when it goes red and the light starts fading he is almost dead and you know you are doing the right thing
Dont use the gun that changes vr-11 as this doesnt kill him just makes him go for a bit.
Obviously the more people you have in the game the more health he will have but then you have people to revive and more firepower lol, i think the best is to have 2 people as he has low health and you have someone to revive you if you go down.