Originally Posted by Skititlez
I actually did this just to see the other ending on N+ and was able to simply "Continue" from the main menu. So yes, the achievement wouldn't pop if you chose that ending, but it's not going to keep you from going back and completing it.
Also, x DIAMONDZ, have you completed all of the co-op achievements yet? I'm off today and would like to grind out the rest of them if you need a partner.
Add me, we can do some as I don't have any partner.