Probably the harshest way to kill someone is by killing them instantly with a one-shot kill weapon right after he/she spawns. For example, when I'm snipering on top of the building with the helicopter in parkour city (since there is a spawn point there), I check every now and then to see if anyone will spawn right beside me there (especially after I kill someone). As soon as someone spawns, I kill that person instantly. That person has just been killed twice, thrice, or even more times in a row lol. Also, be sure not to be in a position where someone can see you instantly right after he/she spawns. It's a case of being totally aware.