you do know they have 3 seconds until they respawn and another 3 for the spawn shield by then your accolade time has finished.
One kill. First guy dies. Next guy runs up. By this point, the first guy is halfway through respawning. Therefore, you are halfway through your accolade time. You kill the second guy. [Double Kill] The first guy has respawned. His spawn shield is ticking as the second guy respawns. The third guy rushes in and you kill him, getting you a multi kill. The first guy is done with his spawn shield and the second guy's spawn shield is halfway through. The fourth guy runs through and you get an ultra. By this time, the first and second guys have fully respawned and will be rushing you. You kill them and get an unbelievable. Possible? I think so.