Prodigee VisionN's solution is correct, however I just wanted to add that you can make this easier by not bothering with the Kestral (melee boomerang) and instead get yourself the following:
First off, get 15k credits. Easiest method is to farm Dark Sector defense nodes. You can find one on earth and one on Mars that are easy enough to unlock. Make sure they say "Dark Sector Defense" and not "Dark Sector Conflict" as the latter will take you to a PvP game.
After you get your 15k, you can buy the MK1-Kunai straight from the marketplace, or you can purchase from your arsenal screen. Keep it as your secondary, and continue playing the game.
Second, you are going to need materials, as well as credits, for the blueprints. Once again, the Dark Sector defense missions are the best for this. you can earn anywhere between 12k and 20k depending on how many waves you go through. Most people (for whatever reason) won't go beyond 15, but once in a while you will get a few people that will want to go further, and that is just cake considering the higher level the enemies, the faster you will level your gear.
Mars will a ton of salvage, and a good amount of morphics, which you will need for both the Hikou and the Kunai. Earth will give you the rubido (for the Hikou) and the ferrite (for the Kunai). The last bit is plastids (for the Hikou) and nano spores (for the Kunai), which can both be farmed anywhere on Saturn. If you like the defense missions, go do more of them there.
Once you have the credits you need (60k total) and the material (above) you can go to the marketplace and under Secondary Weapon Blueprints
you can buy both the Kunai and the Hikou. At this point your MK1-Kunai's should be fairly close to maxed (level 30).
After you purchase the blueprints (and assuming you have all the material) you can then build them in your foundry. Constructing both weapons will cost 15k each (why you need 60k total - 15k for each blueprint + 15k for each weapon construction). They will then take 12 hours to build. You can rush this for platinum, but I am cheap, so I didn't buy any (they cost real money
Once both weapons are constructed they can be collected and, if you have the inventory space, they will be available for you to use. Then it is just rinsing and repeating the exp farms you are comfortable with (whether they are Dark Sector defenses or something else).PLEASE NOTE
: If, for whatever reason, you sell a maxed Kunai or Hikou before you have this achievement, you will lose progress towards this achievement! I can not stress that enough. I maxed the Kunai, and was working on the MK1-Kunai when I needed space for my Hikou. I though "I already maxed that one, I don't need it anymore". I WAS WRONG
. I watched my achievement percentage for shuriken mastery go from 33% to 0%. All 3 maxed throwing weapons need to be in your possession, whether in your inventory or in your hands, in order for the achievement to pop.
I can confirm that now I have the MK1-Kunai, and the Kunai maxed (again) and my achievement percentage is at 66%. Don't make the same mistake as me - do not sell any maxed weps until the mastery achievements unlock.
Questions, corrections and neg vote justifications below.