A few things:
Zen Headbutt does the same amount of damage to a Fighting-type as rain-boosted STAB Waterfall. Earthquake is also a less situational move than Bounce, since it hits Dialga hard and Grass Arceus is neutral to Head Smash. Also, Waterfall is generally preferred to Aqua Tail; the consistency and flinch rate are generally more beneficial than the slight boost in power. The set should look more like-
Relicanth @ Choice Band
Adamant 4 HP / 252 Atk / 252 Spe
- Head Smash
- Stone Edge
- Jolly Relicanth is not going to outspeed +1 Kingdra since Kingdra will have Swift Swim too.
- Kabutops is not an Extremekiller Arceus counter, and neither is Relicanth. If they can't take it out in one shot, they both die to +2 Earthquake. That said they're both good at revenge killing weakened Arceus.
- No sane Deoxys-S runs Energy Ball.
- You forgot to mention that Groudon completely hoses you by itself.
- CB Kabutops is definitely the better lategame cleaner. While Relicanth is much better at wall breaking, Kabutops is much faster (outspeeding Choice Scarf Palkia / Choice Scarf Genesect is very relevant), and its Waterfall is much stronger, which is very important for cleaning up lategame.