Just set up a small zone on the map and have a NPC wander around with way points. Whenever your PC and the NPC are in the zone at the same time, he will start attacking you. Everywhere else, he will just ignore you. Use a trigger to set it up:
Conditions:
Player 1 has more than 0 alive at zone 'attack'
Player 3 has more than 0 alive at zone 'attack'
Actions:
Change units Player 3 to Player 4
Keep player 3 set as a neutral team and player 4 as an enemy team.