Warframe

You can buy the Sentinel blueprints for 100,000 credits apiece. I decided to go with the Dethcube. The thing pretty much levels itself up. Once equipped, just continue playing the game and it will be leveled up in no time.

Side-note; The Dethcube's weapon is considered a machine gun, so if you haven't yet leveled up a machine gun to level 30. or want to use this one as part of your five, do it.

Update: They have updated the game and the Dethcube's weapon will no longer count towards the Rifle Proficiency achievement.

What i did, and it's working with all the mastery achievements, is when you are close to level 30 with any weapon/sentinel go to any mission and immediately when you hit level 30 close game. Repeat that process 3 times and you will earn the achievement.

You can check the progress on the achievement tab to see if it works :).

***This has been fixed in update 17****

You can potentially get this achievement along with the proficiency achievement if your sentinel hits 30 in the results screen ONLY. If your sentinel levels up while doing the mission to 30 this will only reward you with 1 unless you dashboard.

PS. Your kubro counts as a sentinel.