To me, the Pokemon was too slow, did not have Stealth Rock, and it is quite susceptible to Burns. It really had nothing to hit Rotom-W with reliably, except ExtremeSpeed.
It has to compete with Lando-T, who is "faster" as it offers more immediate power, has a good ability, can escape an incoming Rotom-W with U-turn, and can set-up Stealth Rock. Indeed, they are two different Pokemon with different niches although they have some overlap (such as Earthquake and Stone Edge user as an Adamant Zygarde offers as much immediate power as an uninvested Lando), but the metagame has judged that Lando-T's niche (Stealth Rock setter, Excadrill counter, and bulky offensive pivot) is more valuable than Zygarde's niche (weak priority revenge killer and bulky sweeper with a 100% accurate Stone Edge). Using Zygarde constrains you from using him in the early and middle game when all its counters are weakened for a Coil sweep.
Um, what was the point of comparing Zygarde to Landorus? In fact, what's the point to comparing Zygarde to any pokemon? Zygarde's moveset and stats are perfectly made for its unique tanky sweepy niche and really no other pokemon does it better. If it had stealth rocks, I don't think I'd use it as then it would try to compete with pokemon that probably do it better. It's not slow either, just not blindingly fast. It outspeeds a lot, and the only things it doesn't outspeed are typically pretty frail, and susceptible to extremespeed. The burn thing hurts it, but the only real WoW user I see sticking around very long is Rotom-W (I say "only", even though that one is enough), and scald isn't nearly as popular as it was last gen, so burn's not that huge of a detriment. Its big flaws are a unique playing style that most people are too lazy to learn, and a weakness to special attackers that outspeed him.
Because of this, I'm going to try to make a list of checks, and the best way to handle them with different sets. I'll be adding them over the next few hours and I'd love suggestions.
Rotom-W - Always switch out if possible. Taking a WoW is not an option, unless you already more than +2 atk or something
Greninja - You need 1 DD to outspeed and OHKO with earthquake, or 3 Coils to OHKO with extremespeed. If you don't have either, switch out if possible
Gengar - Only 3HKO's, while you 2HKO without boosts. If you don't have any boosts yet, you might want to switch to a special wall, because you'll take a lot of damage, but stone edge is more than enough to beat this pokemon
Weavile - You need 2 speed boosts to out speed, and 3 atk boosts to OHKO with ESpeed, otherwise, you'll probably want to switch out.
Noivern - same as weavile, 2 DDs or 3 coils needed or you're OHKO'd
Skarmory - He's probably gonna whirlwind you, so just use stone edge to give some damage before you go
Gliscor - Just switch if possible, it takes 4 atk boosts to 2HKO, and he's probably just going to toxic stall or 4HKO you with EQ
Azumarill - CB sets OHKO you unless you have +2 Def, so switch out unless got 2 boosts off already. If you do have +2 Atk, just EQ
Clefable - You can 2HKO him by just spamming EQ, but you'll take up to 70% damage, so consider switching out
Slowbro - you're 2HKO'd by ice beam, and you need 2 atk boosts just to 2HKO. If you magically got 6 coils off, then you outspeed and OHKO, but otherwise, either prep your revenge killer or switch out
Dragonite - If you have +4 atk, stone edge for the OHKO, if you don't, then you're in trouble. DD sets will take up to 82% damage on the turn you break Multiscale, but you don't have have to worry about being outsped if he go's for the DD instead. Coil users will get out sped after one DD, don't have to worry about getting OHKO'd unless he has more atk boosts than you have def boosts. This match up is usually determined by how smart your opponent is. If he go's for the quick KO and fails, you'll win (but take heavy damage in the process). If he tries to boost when you already set up, you'll win. If you were weakened by your last opponent or you couldn't get a couple boosts off, you're screwed.
