Beat Krew in weapons lab was a mission in Jak II. Jak met Krew at the weapons factory, who explained he was working with Baron Praxis on wiping out the Metal Heads by cracking open the Precursor Stone using his newly built piercer bomb. Jak then fought Krew, using his newly acquired gun upgrade.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Who Knew Krew Blew?". Completing this mission will reward you with a silver trophy.
Just as the previous two this boss fight exists out of three phases, each one corresponding to a section of the boss' health bar.
Krew will release many Krew clones; little automated blobs that look like Krew. They are electrified so don't touch them or they'll hurt you for some minor damage. To make fighting them super easy just stand in the doorway and fire the Scatter Gun when they get in range. It should have enough spread to cover the entire doorway, meaning you won't have to frantically run around trying to avoid them. The crates around the arena will respawn on their own. An additional tip for the entirety of the fight is that it is best to avoid standing in one spot or the Krew clones will surround Jak very quickly and it is also best to stay away from the doorway as Jak can fall down the elevator shaft.
When you've taken out enough clones Krew will come flying in and fire three shots at you in quick succession. Dodge these then get to work on hitting him; preferably with the Blasters punch plus fire combo, although the Vulcan Fury and Peace Maker are effective as well if you have the ammo.
A repeat of phase 1, except with even more Krew clones. There's not much to it up until Krew shows up, who just as before is susceptible to your long range weapons.
The last phase starts off with a huge amount of Krew clones coming right for you. Using the doorway tactic this makes it very easy to get through it, but if you run out of Scatter Gun ammunition for some reason you'll have to rely on the Vulcan Fury or Peace Maker.
Krew will once again come closer to the arena, shoot him the last couple of times to defeat him.