after Befriending Lugia
Meteor cave will open up.
Buy friend areas. The last dungeon is the deoxys one
19 deoxys is on the the 20 (the real deoxys)
Fight him alone with friend bow
i dont think you can
Go to Shimmer Desert floor 20 and you can meet Deoxys(You must bring a Secret Slab.)
First, you have to get a secret slab. Once you have that, go to shimmer hill, 13th floor and find Deoxys, defeat him, and recruit him.
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you will have to go up stairs and if your deoxys changes into attack form level it up and then it will learn zap cannon
you get it by entering meteor cave, which is found by blastoise the next morning after recruiting lugia
go to shimmer hill 17F[make sure you have mystery part / secret slab] and find him ps:he' very hard to recruit
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Get to F17 in Shimmer Hill with Mystery Part or Secret Slab in bag and then walk until you find Deoxys. Good Luck