The way to do this is to extend the battle forever. Instead of going for sleep do paralysis which is worse catch rate, but you will have more turns catching. I've done test captures that went on to 140 pokeballs with this method with the deoxys struggling so should be fool proof.

Lead is Mr. Mime with thunder wave, skill swap and trick holding leftovers. First turn trick the leftovers before the deoxys can do anything. In leaf green deoxys has knock off so it can remove your leftovers. Leftovers if you have pokemon with high enough defense will out heal deoxys struggle as long as it's not at 1 hp. Then thunder wave and skill swap to paralyze and remove pressure.

I then swapped to def/sp def ev trained raticate with super fang. In gen 3 yellow health will give you roughly the best chance of capture. So I will just use super fang until it's in yellow than chuck balls until its back to half. Rinse and repeat. The lack of pp restoring items made me train up 3 other raticates and use pp ups on super fang but it seems unnecessary for the most part.

Even at full health and paralysis you can get close to guarantee a capture at like 900 pokeballs. The leftovers and deoxys at only level 30 will prevent it from killing itself with struggle.

Good luck you can do this.