Although there is no real "trigger" to this achievement. I found one way that seems to get the same Grunt to follow you around multiple cities.
1) Roll up, roll up, kill a captain, any captain will do.
2) Let a grunt kill you.
3) Go to the Avenge mission and instead of recruiting him, hit
to shame him, lowering his level.
4) Find him on a new point on the map and kill him
5) He should turn up as an ambush either during a mission or as you walk around aimlessly.
I've done this a few times, the second time, I even shamed him a second time, this drove him mad and he followed me to Ambush me on my first visit to Gorgoroth, he proceeded to get himself killed by a Caragor. He then respawned again to get killed by a follower of mine.