The bullseye weapon is one of the first weapons you obtain in the game, so it's very easy to obtain this trophy earlier in in the game, not to mention how easy it is to get tagged kills, also.
Firstly, to tag an enemy, aim your gun at them like you normally would and use your secondary fire
, it will shoot a little tag into the enemy, (you'll know if they're tagged as there'll be a little beam going from your gun to tagged enemy) which now gives you homing bullets to the tagged enemy, so basically, shoot your gun
preferably in the direction of the enemy to make it faster, and all your bullets should home into tagged enemy, killing them easily. So simply tag an enemy, and kill him with the bullseye whilst he's tagged, and do this 39 more times.