Part 97: Feinne's Notes: Merit CrackedUnkillable: You Can Call Me Al, Part Nine- Strong Breaker
So yeah, that first way is how you beat Yamato. It's actually even easier than it seems, because if you never leave the center ring Yamato will absolutely never move or act in any real way. So, you don't even need to worry about killing him in one turn. Easy as can be.
If you want to do it any other way, good fucking luck. Norn will Samarecarm things that die in an endless wave and there are other things with Samarecarm that will BRING NORN BACK if she dies. Chances are that everyone Yamato skirmishes will get wiped out in a pair of double Magic Yin Megidoloans. If you're at all lower leveled than Dragonatrix is right now you will get bodied trying to do this straight up.