You're aiming for a power level of 732, that would be maxed level masterwork gear in every slot. But we're not really going to worry about masterwork and concentrate on legendary gear. *First thing is to not spend any shards or coins you get, you'll need them later. Next you need to get to a level 30 javelin, this is to make sure your legendary gear is level 80. Time to get the gear and spend your hard earned loot.
After you hit level 30 go to the store, you can buy garunteed legendary equipment for 5,000 minor shards. The bad news is the gear might not work for your javelin. Either most are universal or I had good luck. I was able to buy 4 of these and 3 worked for my ranger javelin. They also have a buying option for a specific type of javelin for 2,500 but that will only get you masterwork with a chance of legendary. I bought two and got one masterwork and one legendary.
Go to the weekly store and they will sell a few legendary items for 2,000 coins. You can also craft specific embers here that can be used for crating and tomb offerings.
Transitioning to tomb offerings, there are four tombs and each tomb usually gives a specific reward to help you narrow down where you need to go. I say usually because I went to the tomb for components and one of the things I got was a sniper rifle. It DOES NOT matter if you use the uncommon offering or the masterwork you will still get legendary gear. Kind of rewarding you for all your hard work.
Tomb of Artinia - Legendary weapons
Tomb of Cariff - Legendary gear
Tomb of Yvenia - Legenday support gear and universal components
Tomb of Gawnes - Legenday melee weapons and javelin-specific components
So with some luck you can get this achievement after hitting level 30 without grinding for any gear. You will likely need to play a mission to get the achievement to pop, pick a random one and you'll be good to go. It does not seem to unlock doing a stronghold mission.
If you don't get lucky you can go back to the regulator store to craft embers and whatever is needed for another tomb offering or you can try your luck at missions. I played hard missions and got at least one masterwork piece every run. Try to do legendary missions and stronghold missions to get the best chance of top gear. Sometimes I would get 3 mastwork pieces of gear or better a run. You can go to a harder difficulty but I never found it was needed.