Words from out pope Chris. The Cena is timed not kill based. You can advance without killing Cena. In his words "Not killing Cena is pacifist route, for even though he demanded blood you showed honor and did not strike down a defeated opponent. And his respect for that gesture grants you access to the spiritual world."

I did some investigating. Not killing Cena and just sitting around is slower. You do get teleported after a set amount of time, however, it still seems to be faster to just fight. Currently, there is a mechanic that is being worked on that will teleport you when you kill Cena but I will be doing more testing.