Whilst you can theoretically get this at any point at which you have both the Lock-on Laser and four enemies there are a couple of spots that are ideal for attempting this. Weapon boxes are random so for either of these methods you want to find the weapon and then restart the checkpoint so that if you die or fail to get the required kills you can restart the checkpoint again whilst keeping the weapon.
Act 1-6: Darkness - During this stage you have to defend a vehicle from enemies in a tunnel. First take out one of the Jellyfish-L and switch to the Lock-on Laser. When the Jellyfish-L explodes it will spawn a number of Jellyfish-M. Lock onto four of these and destroy them. Jellyfish-L spawn two or three at a time so if you fail on the first you'll have another chance.
The issues with this method are that due to the low health of the Jellyfish-M the AI can easily destroy them whilst you're locking on and due to the presence of another Jellyfish-L you may not be able to lock onto the right enemies.
Act 2-1: River - right at the start of this mission you will encounter a number of Gorgie-USN and Chicane's. Boost forward to get ahead of the AI, lock-on and fire. If you have the right weapons you can also use this checkpoint to unlock Trick Shot, Robots Tend to Blow Up, Two Birds with One Stone and Hole-in-One. There are multiple other achievements that can also be farmed at this checkpoint.