Moves
Dragon Dance boosts Mega Gyarados's Attack and Speed stats, allowing it to take on faster Pokemon better, notably beating certain variants of Mega Charizard X, Mega Charizard Y, and Mega Metagross. Waterfall is Mega Gyarados's Water STAB move and hits Landorus-T, Donphan, Golem, and Crustle super effectively. It also has a very useful 20% chance to flinch. Crunch provides Mega Gyarados with a Dark-type STAB attack and hits Mega Metagross, Mega Slowbro, Meloetta, and Necrozma. Be wary of Thunder Punch variants of Mega Metagross, however, which beat this Mega Gyarados set. Note also that in order to check Mega Slowbro, Taunt must be used. In addition to the aforementioned coverage-dependent targets, Taunt can beat Mega Sableye and stop Pokemon like Zygarde and Crustle from using setup moves. Outrage hits equally as hard as Mega Gyarados's STAB attacks and provides valuable coverage against opposing Mega Gyarados, Dragonite, Zygarde, and Garchomp. In order to beat Zygarde, however, Mega Gyarados must have Taunt so that Zygarde can't use Coil. Earthquake can be used to beat most Magnezone and get a better matchup against Magearna and Mega Mawile. Stone Edge allows Gyarados to reliably beat Mega Charizard Y and most Mega Pinsir, and in the absence of Outrage, it is useful against Mega Charizard X and Dragonite.
Set Details
248 HP EVs help maximize Mega Gyarados's mixed bulk better than 252 HP EVs would. 28 Defense EVs with the given HP nearly guarantee that Mega Gyarados can avoid a 2HKO from Adamant Mega Charizard X's Outrage. Similarly, 44 SpD EVs with the given HP give Mega Gyarados an extremely favorable roll against Porygon-Z's Uproar, avoiding a 2HKO without investing many EVs. 144 Speed EVs allow Mega Gyarados to outspeed Pokemon with base 110 Speed like Mega Metagross and Mega Diancie after a Dragon Dance. The remainder of Mega Gyarados's EVs are placed into Attack to hit as hard as possible, and for the same reason, an Adamant nature is used. Intimidate is Mega Gyarados's most viable pre-Mega ability, as it effectively increases Mega Gyarados's physical bulk by a factor of 1.5.
Mega Gyarados can also run a spread of 248 HP / 44 SpD / 216 Spe with an Adamant nature, which lets it outspeed Greninja after a Dragon Dance, covering for the unpredictability of Greninja, in exchange for the loss in reliability, while still retaining its extremely favorable Uproar Porygon-Z odds. This will sacrifice slight offensive presence and the avoided 2HKO roll from Adamant Mega Charizard X, however.
Usage Tips
Against Donphan and other Counter users, outside of Golem, it is advisable to use Dragon Dance twice before attacking with Waterfall so that Mega Gyarados can OHKO them, thus avoiding a potential Counter. If Waterfall is not being run, Outrage or Crunch will usually 2HKO them +1, but Counter must still be played around. If you end up in a seemingly unwinnable matchup and are running Waterfall, it is advisable to go for flinches, which can allow you to win through luck. This is not a strategy that should be relied upon, however. Versus Crustle, Mega Gyarados should use Taunt immediately, if it is running it, set up Dragon Dance, and then KO with Waterfall. If Waterfall isn't being used, Mega Gyarados must play correctly by using Taunt only on turns where Crustle tries to set up Shell Smash and never attack on a turn on which Crustle uses Counter. Without Waterfall, this matchup is not favorable. If Mega Gyarados is using Taunt and Crunch, to beat most Mega Slowbro, it should use Crunch until Slowbro opts to Mega Evolve, as Slowbro has the Oblivious ability, which Mold Breaker is ineffective against. When pitted against Zygarde, Mega Gyarados should use Taunt, set up Dragon Dance, and only then attack. Provided that Gyarados is using Earthquake, it may also check Magearna by using Earthquake (and not Mega Evolving) on turn one and following it up with a Taunt or additional Earthquake depending upon whether Magearna uses Iron Defense or Twinkle Tackle. In some situations, it may be advantageous to not Mega Evolve, which preserves regular Gyarados's Water / Flying typing. This gives it a better shot at beating Pokemon such as Mega Charizard Y, Greninja, and Sawk, although Gyarados must still be wary of Electric-type coverage.
Team Options
Fast and hard-hitting Pokemon such as Naganadel, Greninja, and Choice Scarf Porygon-Z make for good teammates, as they can deal with various Pokemon that threaten Mega Gyarados like Mega Charizard Y, Tapu Lele, and Genesect, while Gyarados can cover up their weaknesses to Sturdy users. Jumpluff makes for a strong Mega Gyarados teammate, as it can deal with Magearna, Mega Metagross, Landorus-T, and certain variants of Mega Charizard Y and Tapu Lele. Meanwhile, Mega Gyarados can support Jumpluff by beating Pokemon that it struggles against such as Crustle, Heatran, and Naganadel. Pokemon with typings that have synergy with Mega Gyarados's make for good teammates. Grass- and Fire- type Pokemon in particular form a Fire / Water / Grass core with Mega Gyarados and cover its weaknesses to Electric- and Grass-type Pokemon. Examples of such Pokemon include Mega Venusaur, Mega Charizard X, and Victini.