To earn this trophy, you have to kill a Visionary after entering a map without any guns, character trinkets, or slabs. (You can't unequip the machete, Hackamajig, Sapper Charge, or Reprise, so you won't be completely unarmed.) Picking up any of these items once you've entered the map doesn't interfere with the trophy.
I got this by killing Egor at the Complex at night. There are only a handful of Eternalists around and they are easy to avoid entirely, so you don't have to worry about being overwhelmed by enemies. The trophy will pop when you enter the tunnels.