The final story achievement. Pops after the credits roll (which you can skip safely). Way before this pops, there's a boss fight for which I'll take the opportunity to post some hints.
The boss is probably not that hard if you bring the right weapons. If you're like me though and brought a silenced sniper, shotgun and a saw launcher ... well, let's just say there'll be lots of kiting and swearing and probably a pretty damn sore left thumb from all the running.
The boss starts in the center of the room. When you aggro him, he starts to lift a boulder and will throw it at your location, so get moving. He then proceeds to chase you, if he gets close enough he'll do a melee range area smash, knocking you down. If you're not in melee range but in a certain radius he'll jump at your location and do the area smash there. He'll then restart his routine and again throw a boulder and so on. There are no intermissions or pauses in this fight.
His weak spots are the bright red spots on his skin. I mostly used the sniper rifle as it did the most damage, but there really isn't much time to aim - only while he lifts the boulder, and only for one or two shots. I found that you can dodge many of his skills by using the double-jump (
while in the air) but it needs to be timed just so the area damage hits while you jump.
You can stock up on ammo on both platforms in the room, where you can also find three medi packs. Use them only in absolute emergencies (for example, when getting knocked down). I kited the boss back and forth through the room, using double-jump to get up to the ammo refills quickly, then turning and shooting him. Took me about 10 minutes to wear him down.