Bignose spawns in the northwest corner of Cruachan Gorge. Normally, two torcs are alive there at any given time. You have to kill them and keep killing them to give Bignose a chance to spawn. The faster you kill them, the sooner he'll pop, so stand in the spawn area (yellow dots on map below) and kill torcs the moment they appear.

Doing this is tricky, though, because it's hard to keep the entire spawn area in view due to hills. Also, the torcs wander away from the spawn area after they pop (red dots on map below), so when you first get to the camp, you'll probably have to search a bit to find the two pre-existing torcs . After those two initial torcs are dead, you can move to the spawn point and wait for new ones.

While preparing this page we killed Bignose four times. The longest we had to wait for him to pop was about an hour of real time.