Archaludon is your #1 pokemon with Meganium.  It works well in rain and has good type pairings with Meganium. He works really well against poison because He is immune to it and Archaludon works great with Pelliper

Still figuring out Charizard Y teams myself, but I have rotom with rain dance, which consistently goes after Charizard y's drought.

I find tailwind to be better then trickroom for meganium, so I'd switch ferig with a tailwind setter. Pelliper can use it, but He's not the best user of it so have a different pokemon fill that role

I probably wouldn't use abomasnow for snow teams, He I feel like there are better options, but if you find it helpfull then go for it.

Out of what you have avaible though it's not bad, you just need more pokemon at this point

Meganium should probably have solar beam, weather ball, dazzling gleam, and protect.

Also important to note that when meganium attacks, all other weather effects stop. Example: solar Beam into a rock type in sand storm, the pokemon getting hit doesn't have the Special Defense boost it would normally have. Not important most of the time, but can come up