You can get this in a couple minutes if you have:
a) Loki or Iwara
b) a bow weapon
c) an ODD key (Orokin Derelict Defense - you can buy the key blueprint in the market and build it in the foundry)
I used Loki and the Rakta Cernos - ideally you set up your Loki/Iwara to stay invisible as long as possible and your bow to do max damage for one shot kills.
Load up the ODD mission and you'll see there is a zipline pretty close to the pod you're supposed to be defending. Now simply stand under the zipline, use Loki's Invisibility or Iwara's Prowl, hit to jump up onto the zipline and use your bow to kill all the infested enemies that will be swarming the pod. When your Invisibilty runs out simply jump down and rinse and repeat until you have your 30 stealth kills. Once the achievement pops either just abort the mission (which lets you keep your key) or play on and you will be able to extract every 5 waves.
Good luck Tenno.