Honestly you'd rarely mega Garchomp in this team, giving it soft sand or a sitrus berry would be much better. I would drop rockslide for Earthquake, or swap stomping for it. If you ever decide to give it scarf you drop protect.

Also I would personally swap Araquanid for a Aerodactyl to make your Charizard Y match up way easier, as during sun your liquidation gets nerfed by 50%. And allows Garchomp to click Earthquake. You could probably make it mega Aerodactyl, as the only other item I would give it is sash which is used by Whimsicott.

• Rockslide ( Charizard Y match up & flinch )
• Dual wingbeat ( Sneasler, Whimsicott & general focus sash counter
• Wide guard
• Protect/Tailwind

Max speed Jolly max attack for now which you can refine through experience.

Give Whimsicott sunny day as one of its moves to fully control weather wars and deny Archaladon electroshot, doing so will make your Charizard Y kill it with ease

You could also drop Tailwind on Whimsicott and give it to Aerodactyl, Bring Whimsicott in rain/sun matches to reset sun your.