For those who don't want to deal with getting this late, or risking experienced crew: This can be done the first mission on a new campaign.
Simply do one of the easier missions of the 3 starter missions, completing at least the main objective. ( supply drop was easy ).
Once you have met the main requirement, send your bomber back home.
Get close enough to tag the airbase for landing, but keep the landing gear up.
Once over your airbase, you are safe from any attacks, so start climbing to the highest altitude. The plane can still randomly break down though.
Send your 5 victims to sit on the wings outside the plane. Fast Forward time with LB, and you will circle the runway high up, and eventually they will die from exposure.
As soon as the 5th person keels over, send the plane to low altitude, and lower the landing gear. Barring no hydraulic/electrical failure, it should simply auto land and the achievement will pop after the debriefing screen.
EDIT: As APEX and SGT mentioned in the comments.... it helps odds of success if you buy the 2 survivors the thermal underwear set. The randomness of the game can have them dying due to cold/O2/breakdowns......
Of course, this is easier accomplished as you progress in the game and get skills like " emergency dive ", but since the nature of the achievement is killing off 5 crew, you might want to do it early on.