There is no code for deoxys.
It is obtained by beating the scores of the Rayquaza's Balloon Panic. Once you have the final score, Deoxys is unlocked and playable. In the first part of the attraction, Deoxys is very maneuverable so you should be able to get the vast majority of the targets without too many issues. Try not to fall off or hit Voltorb/Electrode. In the second part, Deoxys target covers about a quarter of the screen so you will be able to get most of the targets. However, you have to think strategically so not to get the X targets which make you lose 300 Points.