Due to major internet issues over the last two days, I only just managed to complete the challenge.
As I suggested in another thread, QMI-Arc Grenades, Geth, Glacier. Had trouble controlling the spawns for a bit, as I was doing it solo (public match however), but since I have this character with the duration TC I could move around a bit and find large enough spawns to nuke.
It got a lot easier once other team mates joined, as I could guarantee the spawn would be at the bottom of the stairs. Had to use a Cobra missile for the final killstreak, as a Geth Prime was in the middle of the group.
Also had vorcha flamer/cluster grenade support, which was nice. Whoever they were, while they didn't get a killstreak, it wasn't for lack of trying. I must have seen at least half a dozen kill feeds where they got four enemies with a cluster grenade or flamer. I'm sure if I hadn't gotten them done, they would have in my place.
Also got my shiny Valiant V, which I am very happy with.