Charizard Y is an amazing wallbreaker, not a lot of pokemon can switch into it safely, but there are quite a few pokemon that can annoy MZardY to no end and they are:
1. Chansey
2. Garchomp
3. Zygarde
4. Toxapex
5. Salamence
Out of those pokemon, Screech Dugtrio can deal with Chansey, wreck toxapex and provided it switches into full health, it can weaken both Garchomp and Toxapex for a sweep so it is a very fun partner to mega zard y.
Of course, Duggy and Zard need a defogger since they are so weak to SR that it is not even funny, so here comes Tapu Fini, AKA the best defogger in the tier also possessing a very nice typing synergy with zard and duggy.
Ever heard of Tapu koko? ever heard of Protean Greninja? Ever heard of a lot of strong special attackers that can wreck this 3-pokemon-team? Out comes magearna to saves your skin, possibly the best special wall in the tier that can fit on offensive teams.
Now every good team must have a good stealth rocker, the choice was obvious to me, landorus-t forms a very fun defensive core with magearna and tapu fini, I opted for the defensive route to increase its ability to deal with the annoying physical dragons that can threaten this team.
Now this team sounds almost complete, A very strong wallbreaker, a strong defensive core of lando-t/tapu fini/magearna, everything is made to support our beloved wallbreaker, now I need one last pokemon, opposing Zard Y, MegaGross and set up sweepers can be annoying to face, also every good mega zard y team needs a pokemon that can abuse the wallbreaking abilities of mega zard y to sweep late game, here comes garchomp which fits a lot of roles wrapping up this team, it functions as:
1. Check to both Mega Zard Y and MegaGross
2. A way to deal with set up sweepers
3. A late game-sweeper
4. A scarfer sitting at a very good speed tier
5. Ground spam with both Lando-T and duggy
I could go on and on but you get the idea.
Some set notes:
1. Rock Slide is preferred over Stone Edge as its most targets anyway are 4x weak pokemon in which the accuracy matters more.
2. Toxic is preferred as a panic button to deal with walls on Duggy but Stone Edge can also work as an EdgeQuake covers which can be helpful in many occasions, there's also sucker punch which is.. sucker punch
3. While Dragon Pulse may seem like a weird choice on Zard Y, let's keep in mind that Focus blast's two targets (Heatrap and Ttar) get wrecked by duggy and they are checked / countered by most pokemon in my team anyway, I've slashed it with focus blast as an option to use as DPulse can deal nice damage to the dragons threatening this set.
4. Ancient Power sounds like a very gimmicky choice at first, but it really helps with beating opposing mega zard ys, mantines and dragonites (even though those are rare, but the point is in beating mega zard y which can wreck my team), anyway I've slashed it with both Fblast and Dpulse as other choices