Complete a mission, contract, or stronghold with a masterwork javelin rarity.

Javelin rarity is determined by the type of gear you have equipped. You need your javelin to have a "gear score" of 486 or higher to reach Masterwork rarity for this achievement. Item drops are determined by the javelin type you are currently using, so try to pick a class and stick with. Switching between the various classes will slow the leveling of all of them.

There are 11 gear slots to place equipment in, and you must have 10 of them at Masterwork quality (45 points per Masterwork item = 450 gear score) and one of Epic quality (36 points for an Epic item = 486 gear score) at minimum to unlock the achievement. You could obviously get a full set of 11 Masterwork items to get this more easily, and any Legendary items you find will help slightly as well, though these only have a score of 47, a two point increase over Masterwork.

As mentioned in the Road Map, difficulty will affect the "rarity" achievements. All other achievements in the game can be done on the easiest setting, but item drops are affected by the difficulty you are playing on. Once you hit Level 30 (your class level is different from your javelin rarity) you will unlock Grandmaster difficulty. Turn this on as soon as you can to increase the chance of better drops (and play on the highest difficulty you can muster prior to this as well). This should get you purple Epic gear fairly often and raise your javelin rarity to over 400 with a full purple kit.

Once you have a 400+ javelin, you should be ready to tackle the game's Strongholds. These are currently the toughest encounters in the game. You will get a guaranteed Masterwork drop from the final boss of these on Grandmaster difficulty (don't increase to GM2+ despite the better drop rate, they take much longer to complete this way). You may also get another drop from enemies or chests. Continue to run these at about a half hour each with a good team until you're filly kitted in orange for the achievement. Tyrant Mines is the suggested Stronghold to farm.

If you have Loyalty Level 3 with a faction, you can also find Legendary Contracts to do, which also guarantee a Masterwork reward. Do these alongside your Stronghold grinding to break up the grind a bit and see some other content. And remember, just because you're guaranteed a Masterwork item, doesn't mean it will be for a slot you need. You could constantly get weapons when you need to fill other slots. There is plenty of RNG involved, but just keep grinding for the win.