I want to get reshiram/zekrom from the Cave, in Omega Ruby, to put it in perspective, my highest level Pokemon is level 80. Can you tell me where to train or what item to use to get him all the way up, or another strategy, whatever works best.
The fastest way are the Blissey base. They are basically triple battles against level 100 Blisseys that will never attack and have toxic orb so they're guaranteed to die. You just have to scan a few qr codes in your own secret base to add them to your game, or something along that line. And make sure you farm lucky eggs from I believe it is Pelipper that holds them. Use the dexnav for that. Read that link for more details
Now, other methods include
Increase your Pokemon's affection in Pokemon-Amie. Another way to get extra experience points from any Pokemon in your game is to increase the Pokemon's affection in Pokemon-Amie. Play games with it, pet it, and feed it until the hearts are filled up. You can check a Pokemon's Pokemon-Amie stats when you try to switch to another Pokemon at the Pokemon-Amie menu.
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Use the Exp. Point Power. If you use this O-Power, Pokemon will obtain even more experience points after battles, but only for three minutes after using the O-Power. This O-Power can be obtained in the Mauville City Pokemon Center. Keep using it and the Exp. Point Power level will increase, allowing your Pokemon to earn even more Exp!
When it comes to finding the right battles to earn a ton of experience points, you can always rechallenge the Pokemon League. In Omega Ruby and Alpha Sapphire, you can now easily skip the credits! You can also challenge a few masters around the Battle Resort, but they only take challenges once a day
Another option is to look for a Mirage Spot with wild Audino. Audino give a great amount of XP each time you defeat them
