To King Ghidorah's credit (Well, Mecha King Ghidorah, here.) , it does seem to have some options:
1. Gravity Beams & Class M Lifting Strength vs Deoxys's Unknown.
Could it pin/immobilize Deoxys & render it incapacitated or exhaust Deoxyss stamina via Gravity Beams, or physically?
(Stamina is Ghidorah's unjustified, keyless
Stamina: Extremely high vs Deoxys's
Stamina: Very high (Able to use extremely powerful techniques without noticeable exhaustion and it can directly fight against even other Legendary Pokémon, such as Rayquaza).)
2.
Energy Manipulation and
Energy Absorption, apparently, might be useful against some of Deoxys's attacks, like, presumably, Hyper Beam.
Does Ghidorah get any benefits, like Statistics Amplifications from absorbing energy? Can it handle absorbing energy from something comparatively stronger than himself like Deoxys? (Also, are all of Deoxys's forms higher in AP? How do Attack Forme, Defense, etc., mathematically compare here?)
3. Mecha King-Ghidorah has Time Travel, apparently. How does he use it, if at all?