These three NPCs are very aggressive and will start shooting you as soon as you get close so make sure you gear up before going after them.

You can find Raptor at the downed plane to the south east of Coral Castle. He throws grenades at you with deadly accuracy.

Zenith is at the weather station on the mountain to the south of Catty Corner.

Blackheart is chilling out on the ship to the west of Holly Hedges.

You only need to eliminate one of these so I would recommend you go after Blackheart. You will be able to land close by with enough time to gear up before you engage.