Basically you have to kill all four enemies before the first one respawns. So that the enemy team is all dead at the same time and you were the one to kill them. It's defined as wipe out an enemy team with at least an overkill. Meaning it has to be at the same instance with the entire team dead before one of them respawns.
You'd literally have to kill the entire team in one action, I.E., one sniper round through all their heads, a grenade, rocket, IC round, spartan laser, rail gun, something like that. For slayer anyway.
For flag or non instant respawn games it'd mean you have 6 or 7 seconds total to massacre the entire team.