Just tested it my Azurill had max happiness and it evolved after I used a rare candy.
@EffervescentJoy if you want you can upload your save file and i will tell you what the happiness of your Azurill is.
I already tried an eeveelution team and tbh it is not rly a challange since they are all good. Also you don't need a HM slave if you know how to change some scripts in reborn. I made the player do all the cutting and smashing
I think you need to have done the daycare event or something else... cant remember. But if you miss it now you will be able to buy it later in 7th street.
You can get one beneath the grand stairway. If you remember the 4 paths where you can jump down, take the second from the left, and smash the rock just under it and it should be behind it.
That will be an easy battle for someone who has good ev/iv pokemon if you compare for example a ev trained/good nature/iv's gyarados with a untrained mega gyarados, the trained one is stronger
It is much easier if you use a script to change the pokemons form, that way you can have it mega evolved even before the battle starts and it can hold any item, no need for the mega stone.
If you really want to make it harder do the Little Cup challenge (play with the lowest form of a pokemon that is able to evolve) I am currently at the sixth badge and it is really hard even with perfect EVs and good IVs