You get 4 pieces in total from the "Morale Breaker" achievement, the achievement is to do the strike 11 times. You get a reward each time you do so, and you get a piece when you do it 5 times, 8 times, 10 times and 11 times. You can get 2 pieces from the Reward tracks if you want. The drops are just there for people who don't do the strikes. You're talking about the tiers of the achievement. You actually have to complete the strike mission 50 times. Tier 5, 8, 10 and 11 need to have 20, 35, 45, and 50 completed runs respectively.