Q&A for How to Catch Deoxys in Pokémon Omega Ruby and Alpha Sapphire

You can find it in the aqua/magma hideout. It's around the area with the 4 pokeballs altogether. 2 are elctrodes and one's a master ball.

You could try to win one in the ID lottery In Lilycove City, or get one from someone else's secret base.

Any Pokeball would work eventually, but a Master Ball would work the best. If you don't want to waste your Master Ball, I would use an Ultra Ball.

Yes, you do have to beat the Elite Four in order to catch Deoxys.

Your only option is beating the Elite 4 again, because the game saves after defeating/catching Deoxys.

You can get a timer ball from the Rustburo lab near the Pokeball table. If you talk to the scientist there, he'll give you a timer ball.

Yes, you can get more than one Master Ball.

You could get it by doing the Lotto ID at Lilycove, or trade for a Pokemon holding one.

Use a level ball (while the Pokemon with a level that is four or more times greater than the Deoxys) while Deoxys has only one HP left (you can do this with the move false swipe so you don't faint it).