Added, grats!WOOOO Finally did Redundancy!! :D
Obviously, you don't need achievements that are now unavailable. As for the OMotm, I won't count it, since they also become unavailable like Camomons.Also, I have a few questions: To get the custom title, do all achievements include omotm and other achievements you can't get anymore? Also, does it include the any om achievements? If it does, do you have to get each of them three times or just once?
You do need the Any Other Metagame achievements, but you only need to get an achievement for one metagame.
edit: added Ransei's achievement suggestion, Lonely God; check it out on the OP if you haven't seen it.