You will get this after either watching the credits or skipping them. I choose to skip them and it popped at the title screen. Credit to PhantomSiren, who has the solution for uber, currently if you play this chapter on Uber you can get all the difficulty achievements or if you crack enigma 1 you can do Uber in 999 mode(infinite guns, 999 health).
Redid tips after doing it on Uber:
Before going down into the hole grab any grenades extra ammo, health and armor you can.
If you reload from checkpoint it seems to magically lose rocket ammo, I had full rocket ammo, then upon reload only 8.
What I did was lob grenades to stun him and blast with dual lasers.
Then run to the back of the area, use a full LFK blast to stun him then blast, then blast some rockets.
Repeat the stun and rockets. If you hit with about 12 rockets plus the grenades and the other weapons he should go down.
I made this much harder than it had to be(thanks qG Lecca), it seems like three stuns plus a lot of damage during does the trick.