**UPDATED WITH NEW REQUIREMENTS**
---With the release of Operation Phantom Sight on 6/11/19, several achievement requirements have changed. To get this achievement, you now have to get 5 kills with SMG's.---
You will likely get this while defending, although it is possible to use certain SMG's on attack. Since the achievement is self-explanatory, this solution will go over what guns count as SMG's, and tips on getting kills with them. First off, these are all of the SMG's available in the game as of Operation Phantom Sight, along with the operators that are able to use them (Attackers will be in bold, Defenders will be underlined)
1. FMG-9 (Smoke, Nøkk, Recruit)
2. MP5K (Mute, Recruit)
3. UMP45 (Castle, Pulse, Recruit)
4. MP5 (Doc, Rook, Recruit)
5. P90 (Doc, Rook, Recruit)
6. 9×19VSN (Kapkan, Tachanka, Recruit)
7. MP7 (Bandit, Recruit)
8. 9mm C1 (Frost)
9. MPX (Valkyrie, Warden)
10. M12 (Caveira)
11. MP5SD (Echo)
12. PDW9 (Jackal)
13. Vector .45 ACP (Mira)
14. T-5 SMG (Lesion)
15. Scorpion EVO 3 A1 (Ela)
16. K1A (Vigil)
17. Mx4 Storm (Alibi)
18. AUG A3 (Kaid)
19. P10 RONI (Mozzie)
As for which gun to use, I HIGHLY recommend the 9MM C1. It has an insane damage and range, even outgunning several assault rifles in those categories. But if you don't want to use that gun (or if you don't have Frost), there are plenty of other options. The MP5 and the MP5SD have the ACOG sight, which can help you hold longer angles (the P90 has the ACOG as well, but I would advise against using it. It has the lowest damage out of any gun in the game, and the recoil is uncontrollable). Guns like the UMP45, 9×19VSN, and the K1A have little to no recoil, which allows you to hit your shots, while guns like the MP7 and Scorpion have high fire rates, which allow to kill faster. No matter which gun you choose to use, you only need 5 kills, and remember, headshots are always instakills.