I got to Rank A, using some form of Zygarde the entire time.
I initially started using Jibaku's AV set along with a Mega Metagross and a Glaceon. I liked the utility Thousand Waves provided, trapping foes in so that my Metagross and Glaceon could pick up the kills vs whatever was trapped. Used that for a couple matches.
I then wanted to try Mega Zygarde, because I wanted to send people to the Shadow Realm. After some set tweaks, this was what I came up with:
Zygarde-10% @ Zygardite
Level: 50
EVs: 4 HP / 4 Def / 252 SpA / 12 SpD / 236 Spe
Modest Nature
- Endure
- Protect
- Core Enforcer
- Earth Power
I was initially running my own Mega Zygarde with a screener, but I wasn't doing it right, running a Clefable with both screens. It simply wasn't explosive enough. Theorymon definitely has the right idea where an offensive screener like Delphox is the wave. Mega Zygarde already needs a ton of setup, and spending time just throwing out weak Moonblasts wasn't netting me points.
Instead, I took a different approach, and used SD Charcoal

and CM Life Orb

instead, two offensive threats that consistently got kills while I charged my mega meter. Since I wasn't running screens anymore, I thought of Endure AND Protect on my Zygarde as I mentioned in my earlier post, and I quite like the safety nets those two give me. Endure gives 10% Zygarde the path to Complete form, and Protect is a secondary shield when things got hot.
The EVs are where things get interesting. Max SpA scores the cleanest OHKO on a Pokemon I've ever seen in my life:
252+ SpA Zygarde-Mega Nihil Light+ vs. 252 HP / 4 SpD Xerneas: 234-274 (100.4 - 117.5%) -- guaranteed OHKO
The Speed is the secret sauce. That exact investment lowers Zygarde's cooldowns by a whole second, notably turning Nihil Light from an 11.5 second cooldown to 10.5, and Earth Power goes off every 3.5 seconds. Considering how much of a magnet Mega Zygarde is to attack, I felt every second counted, and I was shocked by how fast my Zygarde was throwing off moves. While my Nihil Light was charging, I used Protect and Endure when necessary to pad out the time for Nihil Light to finish, and I was able to use the laser quite a few times per match this way. I made up for the lack of bulk by always being aware of what's going on around me, timing the Mega Evolution and the usage of Nihil Light just right.
Am I claiming this is the best way to use Mega Zygarde? Nah. Screens + bulk investment seems more consistent and safer, but this was a pretty fun build. Mega Zygarde is definitely a skill mon that tests your knowledge on this game's mechanics, as well as positioning and situational awareness. It's kinda like using a Sniper Rifle in an FPS game. Very hard to use and master (depending on the game lol), but a skilled player will go stupid with it.
I definitelt want to shift to other non-Mega Zygarde sets. That Life Orb set Theorymon posted seems cool! I also agree that Zygarde in general very likely has a lot of hidden tech that'll make it a lot better than we thought!