You cant get Deoxs in Emerald without cheating. But you can trade it off of FireRed or LeafGreen. If you have FireRed or LeafGreen, heres what you have to do.
1. Go to any shop and go to the survey thing where you have to put four words.
2. Then put "Link Together With All"
3. The shopkeeper will say something and you will obtain "Mystery Gift"
4. Trade with another gameboy that has "Mystery Gift" and if you do it right, you will get a ticket.
5. There are two tickets. One will take you two an island where you can catch Lugia and Ho-oh ( Lv.70 ) and the other one takes you too Birth Island where you find Deoxs.
6. You will have to go through a triangle puzzle and deoxs will rise. ( Lv.25 - Master Ball Sudjested )
7. After you catch Deoxs, trade it to your emerald and your all set!
Hope I Helped!
that is a way but without doing that (trading i count as cheating) is you have to get the succesul launches up to 100 at the space station (either that or the number that the guy thats looking out of the window was just put there for decoration) note: i can not test this cause i got to 74# of launches( u need 75# of launches to go to space and get deoxsys/jirachi and u need 50# for ruby/saphire but u cant get jirachi in ruby/saphire) then i lost my ds(i have like 4 ds games i just play my gameboy games on my ds)