I kinda want to reject this mon tbh. Unless the other guy happens to have an Electric Arceus I don't see where this thing gets free turns.It's so slow and fragile.
Outside of Electric Arceus, the biggest one is opposing Ground type. Groudon, Lando, and Gliscor are all quite common and they usually are forced out by Mamo. BTW, I think Sub is the best fourth move because if you sub on a switch, your opponent will usually lose one guy unless he has a bulky arceus (that is not weak to your STAB) alive. I think that often times the problem for Mamo is that if you predict wrong on the switch you are forced out and Sub really fixes that. For example if your opponent has both Kyogre and Lugia waiting in the wing, you can easily sub on the switch and kill both of them by hitting with an appropriate STAB. Mamo will suffer a great deal of residual damages with Sub+Life Orb, but it is usually worthy.
In any case, you and other QC can reject this if you are not convinced. It has been a while already.