This is not as simple as scoring a kill on Colt. He has his own Revivify slab, so you’ll potentially need to take him out three times to grab the trophy.
Colt only has to kill you once.
The sniper/shotgun combo is most effective, going for high, long range kills where possible, and switching to the shotgun for CQ kills.
Players will usually try to return to their body after the first kill, to recover gear and residuum, so putting down a prox. mine on the corpse is a good idea.
This is one of those Trophies that is sooo much easier if you’ve got a friend you can work with.