I've seen the Killstreak medal appear a handful of times and yesterday learned there are actually three levels to it (just like most medals) but have never been able to figure out what triggers them. A multikill of some kind? Or is it a number of kills by the team without anyone going down?
To be awarded this medal, the player's team (yes, the whole team counts, as it is a squad-medal) needs to kill 5 enemies or more in 1 second. So it can be done with a rocket launcher of course or even when everyone of the team kills an enemy and one player has to kill two with one shot (which can easily be done with weapons like the Widow or Javelin) and the shots have to be fired nearly simultaneously!
You get 2500 points for the bronze medal (1 killstreak), 5000 points for the silver one (2 killstreaks) and 10000 points for the gold medal which is equal to 3 killstreaks!
It is awarded for killing a number of enemies in a short amount of time. The name "Killstreak" is misleading. The name is more synonymous of something with something like a Halo Overkill, Killtacular, etc. medal.
The number of required enemies is either 4+ or 5+. An easy way to obtain the medal is killing an Atlas surrounded by Cerberus troops with a rocket launcher.
Another way to trigger a killstreak is by lobbing a lift grenade in a pack of trooper level enemies. I did 3 killstreak in a silver game with my Asari vanguard by spamming lift grenade, which are, by the way, very powerful!