Here's a big helpful paragraph:
Get Diablos to level 99. Now, find the enemy you wish to Devour, and use LvUp and/or LvDwn until they have 9,999HP or less (use a guide or scan them). Now, use Diablos to do 99% damage to their HP (if they're not immune to % attacks, like Diablos and Demi). Now, just Devour them. Devour is like Card, they have to be at low HP for it to work, and you don't recieve any EXP if you Devour all the enemies on the battlefield (which helps to keep Diablos at level 99).