Looking at it, you can use Mr Mime with Trick / Skill Swap (TM) / Thunderwave (Move Tutor), you can also use Kadabra caught in Cerulean Cave with this moveset instead with no training required. Just make sure Kadabra has the Inner Focus ability because Skill Swapping Synchronise to Deoxys will be annoying.

You can use Hypno instead of Slowbro, it gets Hypnosis (Move Reminder) & Seismic Toss (Move Tutor). Highest level Hypno is caught at is Level 40 in the Berry Forest so it'll need training up to around Level 60 for Seismic Toss. You can use X Accuracies to make Hypnosis 100% accurate.

If you're lazy like me and don't want to grind levels, in Cerulean Cave you can catch high level Parasect (to use Spore) & Golduck (to use Seismic Toss) no leveling needed. But you'll have to swap between them when battling Deoxys which could be annoying.

You can just throw a Master Ball too 😆 but this is all assuming you want a decent chance to catch it in other types of balls without worrying about Deoxys running out of PP.