Takes a bit of save-scumming, but I guess the Deltas are doable. I reloaded to prior to the mission and gave each of my soldiers two C4s. When I restarted the mission, the Delta was conveniently killing civilians right in front of me. Took 4-5 C4s to take him out. On a sidenote: tossing a C4 into a fire tile detonates it immediately, so you only need to set off the first C4 with a flashbang.