Colonel Guillermo Tizel is an FND officer and Rambo superfan who has talked the government into letting him keep Rambo's Vengeance Bow. You ambush his fishing trip for the key in the crossover mission All the Blood.
Tizel worships Rambo. Yaran law bans makeshift weapons, so he cannot actually use the Vengeance Bow, but he keeps it locked up anyway. A note suggests he mocked Morello, who failed basic training, which may be why Morello wants revenge.
Hit his fishing trip before he can set up. He buffs nearby soldiers and calls airstrikes, so take him out first and move when you see a flare. He carries the key you need for the bow.