Resolved reincarnation novel

in the first world i remember he died while fighting the strongest person in the world and they died because a nuke was dropped on their heads and it signified a new era.
after that there were a couple more worlds, like a mech world etc.
i only remember the first world though

 

I doubt this is it, Mystical Journey.

 

It's indeed Mystical Journey

 

thank you very much!!!!