Just like the Magnum kills, you're quite easily able to obtain the necessary Bullseye secondary fire kills in the prologue, which if you do not, it's easily obtainable in chapter 1, which is where I achieved it.
To know you're counting for the trophy, you must first shoot an enemy with your homing tag,
. If done and hit successfully, the enemy will glow a brightish green colour, which allows you to see him through floors/walls/from a distance, which all you need to do afterwards is fire normally with
and kill him, all your bullets should go to the tagged enemy, making it an easy kill.
Almost every chimera drops a Bullseye weapon, meaning if you walk over it, it'll give you more primary and secondary ammo, which also means you won't run out of ammo so easily. I got this trophy by the little pipe segment with the flying bots in chapter 1.