You can work on this achievement as well as
at the same time.
The first Ork Nob you come across that you can battle is in Chapter 2 (after the prologue on the Ork ship).
After meeting up with both team members and running the trench towards the bunker, you will come across a group of marines making a stand. After a few dropship waves, an Ork Nob will be dropped into battle.
Repeatedly stun combo him (x,y or x,x,y or x,x,x,y) until you can struggle with him in which you can repeatedly tap b.
Reload the checkpoint until you get the achievement.