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(Ruth)less is More
Collect any class armor set from PvP Season 10.
The '(Ruth)less is More' achievement awards 10 points for collecting any class armor set from PvP Season 10, purchasable with 
Marks of Honor from Knight-Lieutenant T'Maire Sydes (Alliance) or Blood Guard Zar'shi (Horde).
Acquire Marks of Honor
You need either 
17 Marks of Honor to buy all individual pieces (Hands 2, Head 3, Legs 3, Shoulder 2, Chest 3, Waist 1, Feet 2, Wrist 1) or 12 Marks of Honor to buy the ensemble — the ensemble is the cheaper option if you have no pieces yet.
Purchase the Set or Ensemble
Alliance players visit Knight-Lieutenant T'Maire Sydes and Horde players visit Blood Guard Zar'shi to purchase the Ruthless Gladiator ensemble for your class using 12 Marks of Honor; no old PvP rank is required. Using the ensemble immediately adds all appearances to your collection and grants the achievement on the spot.
Tips
If you bought individual pieces instead of the ensemble and the achievement did not fire, the items may have an active trade timer preventing collection credit — delete or disenchant the items to immediately clear the timer, then relog if needed.