UPDATE: With the game coming to an end the best plan now is to use your Eternity Splinters to buy lots of loot boxes. Max out your stash and inventory and buy about 10 to 20 boxes at a time. With those you can accumulate Legendary Marks, materials, credit boxes to level your crafters, along with more splinters to buy more boxes. You could have this achievement in less than an hour this way.
First, if you see a Doop, kill it. They not only drop a lot of crafting mats, but also 2 Legendary Marks every time.
Next you need to donate enough to the Science Division Specialist to get her to level 6, this will unlock the Reserve Modification. Now you need the mats for it: 5 Ionic Particles, 5 Nanotech Filaments, 10 Spare Parts, and a Modification Installation Kit. Now unless there's a way to craft one or they are an ultra rare drop the only way to get a Kit is to buy it from the Logistics Division Quartermaster for 100 Legendary Marks. If you need any of the other mats you can either buy them from him or craft them at the Engineering Division Specialist if he's leveled enough. Finally the cost of crafting it is 100k credits.
Obviously for the credits and the Marks you should do Legendary missions as soon as they unlock after defeating Loki. If you're lucky you'll meet up with some Doops along the way, or get lucky with some of the bounty chests. With your Marks you should buy the cheap Legendary item, Helm of the All-Father, first from the Legendary Item vendor in Avengers Tower/Odin's Palace before saving up for this, since leveling that will take quite some time for the achievement "Myth No More".
If I've missed anything or you know of any info I'm not aware of feel free to comment and let me know. If you give negative feedback, please let me know why so that I might improve my guide. Thanks and good hunting.