probably using a range weapon or something in order to locate him ( meaning, use the range weapon's aiming circle to spot him... ) . Try with some throwing knives or even a bayonet : if u use a bayonet, it will still lock the target since u can use the bayonet's stabbing ability, so all u need to do is spot him, shoot him ( this won't cause any damage ) and he will come to you and attack u, then u will be able to kill him. You can also try with grenades, but this might turn out to be a lil bit more difficult depending on the way in which u use them.
Hope it works and good luck...
P.S: Sorry if this is too late, its the first time i read this new thread.