Moves
Giga Drain allows Mega Venusaur to check Pokemon such as Keldeo, Azumarill, Mega Diancie, and Landorus-T while simultaneously recovering lost HP. Sludge Bomb is a more powerful STAB move that lets Venusaur check Grass- and Fairy-type Pokemon, including Celebi, Breloom, Azumarill, Mega Altaria, and Clefable. It's also a great move to use on a predicted switch, as many Pokemon capable of switching into a Giga Drain and forcing Mega Venusaur out, such as Life Orb Tornadus-T, Latios, Thundurus, Mega Pinsir, and Talonflame, are crippled by Sludge Bomb. Growth gives Mega Venusaur a way to boost its SpA, enabling it to get past Chansey, while also boosting alongside Calm Mind Clefable to shut it down. Alternatively, Knock Off gives Mega Venusaur an immediate way to cripple Heatran, Tornadus-T, and Chansey, allowing it to beat them long term instead of committing to a sweep. Synthesis gives Mega Venusaur semi-reliable recovery, which lets it continually tank hits.
Set Details
16 Speed EVs are used to outpace Adamant maximum Speed Azumarill. Maximum HP EVs are used to make Mega Venusaur as tanky as possible. 8 Defense EVs allow Mega Venusaur to avoid the OHKO from Mega Metagross's Zen Headbutt, and the 2HKO from Mega Diancie's Diamond Storm after Stealth Rock. 28 Special Defense EVs are used to survive Mega Alakazam's Psychic from full HP. The rest of the EVs are put into Special Attack alongside a Modest nature to hit as hard as possible. Mega Venusaur can also opt for 56 Speed EVs to outpace Pokemon such as Adamant Crawdaunt and uninvested Rotom-Wash, or 92 Speed EVs to outspeed uninvested Landorus-T. Alternatively, an EV spread of 248 HP / 104 Def / 112 SpA / 28 SpD / 16 Spe can be used alongside a Modest nature, allowing Mega Venusaur to avoid a 2HKO from Choice Band Weavile's Icicle Crash from full HP and still hit hard. Chlorophyll allows Mega Venusaur to revenge kill Pokemon on the turn it Mega Evolves if it is in the sun.
Usage Tips
Your first goal when playing Venusaur should be to Mega Evolve as soon as possible, as without being Mega Evolved, Venusaur is hit super effectively by Fire- and Ice-type moves and also has far less bulk. Mega Venusaur does have good bulk, but it's imperative not to switch Mega Venusaur into Pokemon, even those handled by its typing, unless there wouldn't be any negative consequences or the situation is dire. Mega Venusaur has the freedom to switch into Clefable, Rotom-W, Keldeo, Alomomola, Landorus-T, Mega Manectric, and Azumarill, but it needs to be extremely wary of getting worn down, largely from the foe using U-turn or Volt Switch on it, burns, and switching Mega Venusaur into Stealth Rock. Because Mega Venusaur lacks Leftovers recovery, it gets worn down quickly from all of these elements, which severely cuts into its longevity. If you can, avoid switching Mega Venusaur directly into anything, and instead check threats when given a free switch. When selecting a move to use, Giga Drain is your best friend; Mega Venusaur is worn down quickly and really appreciates the recovery Giga Drain gives it. If you get Mega Venusaur in safely and see a Flying- or Dragon-type capable of switching into a Giga Drain and checking Mega Venusaur, use Sludge Bomb, as it does a surprising amount of damage to these potential checks. Overall, your goal with Mega Venusaur should be to switch into Pokemon it beats, deal massive damage, heal, and then switch out before it takes excessive damage. You have to be really careful with Mega Venusaur, because if you play recklessly, Pokemon that Mega Venusaur should be able to beat, such as Mega Diancie, can beat Mega Venusaur instead. Growth allows Mega Venusaur to be a solid late-game sweeper once its checks are weakened or eliminated.
Team Options
Mega Venusaur absolutely despises being burned by Rotom-Wash and Keldeo, so Heal Bell or Aromatherapy users and burn absorbers such as Celebi, Clefable, Reuniclus, and Starmie are practically mandatory teammates. Celebi doubles as a counter to both Rotom-Wash and Keldeo, so it also can take the strain off of Mega Venusaur to deal with these Pokemon. Healing Wish Latias can also prove to be a great teammate, potentially healing Mega Venusaur of any unwanted burns while recovering Mega Venusaur's HP. Mega Venusaur also despises having to take entry hazard damage because entry hazards cut into Mega Venusaur's longevity, which helps Landorus-T and Mega Diancie beat it. Entry hazard removers such as Latias, Latios, Starmie, Zapdos, Mandibuzz, and Excadrill can therefore make for good teammates. Psychic-types such as Reuniclus, Latios, Latias, Mega Gardevoir, and Mega Alakazam also threaten Mega Venusaur. Bronzong can wall Latios, Latias, and Gardevoir, while Bisharp can deal with all of them with either Sucker Punch or Pursuit. Flying-types such as Talonflame, Mega Pinsir, and Tornadus-T are also a problem for Mega Venusaur. Rhyperior can deal with the former two as well as Superpower Tornadus-T, while Zapdos can check the latter two. Tyranitar may seem like a poor teammate, due to its sand nerfing Mega Venusaur's recovery, but Choice Scarf Tyranitar is actually a great one, eliminating Latios, Latias, and Starmie with Pursuit. Similarly to Tyranitar, Hippowdon may seem like a poor teammate because of Sand Stream, but, in reality, it's a great one, checking Flying-types and VoltTurn teams while appreciating Mega Venusaur's role as a Manaphy check. Additionally, to bypass the conflict between Sand Stream and Synthesis, try to use Sand Force Hippowdon when you're using it and Mega Venusaur on the same team.
Additionally, Scizor can deal with Kyurem-B for Mega Venusaur, while Manaphy, Slowbro, and Landorus-T can deal with Heatran. Both offensive and defensive pivots are immensely useful, as they make it easier for Mega Venusaur to switch into moves and give it more time to use Synthesis, therefore bolstering its longevity. Slowbro and Slowking are useful as more defensive pivots with Regenerator, Assault Vest Tornadus-T acts as a defensive Regenerator pivot with U-turn, and Raikou, Scizor, Rotom-Wash, and Landorus-T can all act as offensive pivots. Essentially, any teammate capable of dealing with a Pokemon that Mega Venusaur cannot makes for a great teammate, as Mega Venusaur's defensive abilities can prove to be a wincon in some games.