Meganium
Overgrow
Raises the power of Grass moves 50% when at 1/3 HP or less.
Type Tier
Grass UU
Statistics
Min- Min Max Max+
HP
80
 - 301 364 -
Atk
82
 180 200 263 289
Def
100
 212 236 299 328
SpA
83
 181 202 265 291
SpD
100
 212 236 299 328
Spe
80
 176 196 259 284
Name Item Nature

Standard Defensive Meganium

 Leftovers Bold / Calm
Moveset EVs
~ Leech Seed
~ Synthesis
~ Counter / Reflect / Hidden Power Grass
~ Light Screen / Hidden Power Grass
252 HP / 132 Def / 72 SpA / 52 SpD

Counter is great on Meganium, 100 base in both defenses works excellently. For pseudo-passing and the 50% recovering Synthesis the same cool defenses are a big benefit. Leech Seed is a move every Grass-type should use, can give your team healing support. Too bad Meganium doesn't have better offensive moves. Counter works best with Light Screen, as it will protect you on the Special side while you kill them if they hit you physically.

Other Options

Razor Leaf is a shafted move in ADV, but a critical hit can mess up Suicune with Calm Mind. Grasswhistle is a poor man's Sleep Powder, and again it could hit once. Like Milotic and Blissey, Meganium is very durable and won't mind the accuracy drop as much as fragile Sleepers like Breloom and Persian would. Toxic and Thief are nothing new as alternatives. Substitute + Leech Seed is an interesting combination that should always be mentioned, but Meganium is really too slow to abuse it. Swords Dance can power up physical moves, but you're probably better off using Victreebel for the STAB on Sludge Bomb. Giving Meganium Hidden Power Fire or Hidden Power Ice will allow you to cover more Pokémon types. Finally, if you are paranoid about being statused, Rest over Synthesis is an option. Don't try Sunny Day or Solarbeam; use Vileplume or Exeggutor for that.

EVs

If you plan to use Reflect, you'll want to get more Special Defense than suggested above. Alternately, you can use more Special Attack to help a bit with Walrein switches.

Opinion

Strictly to be used in UU competition, as it will get eaten alive in most OU games. Ground resistance is nice, and it's one of the few pure Grass-types. Cool Pokémon, but there are better Grass-types for most teams.

Counters

Venusaur for best counter there is. Steels, Weezing, Gengar and Substitute ruin it badly, too. Tyranitar's Sand Stream ruins Synthesis and the vital Leftovers recovery. Salamence can switch in on Meganium quite easily.

In UU, you'll want to watch out for all Grasses, especially those that carry Toxic or are named Vileplume. Fire-types pretty much prove an end to Meganium. Muk and other Poisons in general will have an easy time switching in and have STAB Sludge Bomb to hit Meganium hard. Tentacruel can turn Leech Seed against you with Liquid Ooze, and set up a Swords Dance sweep, or just straight up Sludge Bomb you.