Battle Moveset

A Good Moveset for Meganium would have to be this:

Attacks:

Hidden Power [Grass]
Leech Seed
Synthesis
Counter / Reflect / Light Screen

Items Attached:

Leftovers

Preferred Nature:

Calm or Bold

Strategy Using Meganium

Meganium is one of those Pokémon who has relatively nice stats, a nice move pool but can't compete in the OU environment because it's outclassed by other Pokémon, most notably Celebi and Venusaur. However, in UU, where you won't be seeing Celebi and Venusaur, it fit's in quite nicely. It has very strong stats in both defences and a nice HP stat which are complemented with a resistance to several common attacks (Ground, Electric, Water as well as the less common Grass) as well as some nice recovery options with Leech Seed and Synthesis. HP Grass is generally it's best offensive move, it's the strongest STAB move available to it and hit's a fair few common Pokémon for super-effective damage. In your final slot is generally a filler, Counter is a nice strong move to catch opponents by surprise, with it's good HP and defensive stats as well as it's recovery moves it can use Counter very effectively and once revealed will make your opponent think twice about attacking you with physical moves. Reflect and Light Screen give a boost to your already good defences as well as helping your team.

EV Corner:

EVs: 204 HP / 152 Def / 76 SAtk / 76 SDef
Bold Nature (+Def, -Atk)

With 204 HP EVs you can get the maximum benefit from Leftovers. The rest of your EVs are better focused on the defences and a little in Special Attack. A little speed can also benefit Meganium, with 20 it can drop over the 200 mark to outrun some Pokémon before they use their boosts (notable Agility) and with a bit more it can outrun some other opponents but it generally shouldn't be a focus.

EVs: 204 HP / 76 Def / 76 SAtk / 152 SDef
Calm Nature (+SDef, -Atk)

Same EV set but with the defences flipped, if you aren't using Counter and have a solid physical wall but need a special wall this is probably the set you'll want. If you're using Reflect this is also a beneficial EV set-up. Once again some Speed EVs can pay off.

EVs: 204 HP / 60 Def / 76 SAtk / 168 SDef
Bold Nature (+Def, -Atk)

Balanced defensive EVs, pretty straight-forward.

Other Optional Sets

Meganium @ Leftovers
Trait: Overgrow
EVs: 204 HP / 124 Atk / 180 Spd
Adamant Nature (+Atk, -SAtk)
- Earthquake
- Swords Dance
- Hidden Power [Rock / Flying]
- Synthesis / Leech Seed / Substitute

A bit of a novelty but a physical sweeper set-up for Meganium. The lack of physical STAB hurts this set, especially in comparison to Pokémon like Victreebel who have a Poison STAB to use as well as a lack of usable move pool. Earthquake is it's best move to use but since Grass and Flying types frequently (and easily) counter it you'll run into problems that only Hidden Power can cover. Synthesis and Leech Seed gives Meganium some recovery to help it set-up Sword Dance and Substitute can protect you from statuses and gives you a buffer against attacks whilst you set-up Sword Dance.

Meganium @ Leftovers
Trait: Overgrow
EVs: 152 Def / 176 Spd / 76 SAtk / 104 SDef
Bold Nature (+Def, -Atk)
- Hidden Power [Grass]
- Leech Seed
- Substitute
- Synthesis / Reflect / Light Screen

Sub-Seed set-up. Since large HP stats and Sub-Seed don't mix too well you can skip HP EVs, it particularly helps to have speed to be able to set-up Substitutes so those spare EVs can be thrown into Speed with the rest in Defences. The last move-slot is pretty much a filler, Synthesis is useful but when you're using a lot of Substitutes and Leech Seeds you may not find it particularly useful in comparison to Light Screen or Reflect. Spikes support is very helpful for this set since Sub-Seed usually causes a lot of switches.

Other Optional Moves

Razor Leaf, Grasswhistle, Rest, Sleep Talk, Toxic, Hidden Power [Ice], Hidden Power [Fire].

Razor Leaf is an option over Hidden Power, it's weaker but the critical hit chance helps make up for the lost power and it frees up your Hidden Power for a different type. Grasswhistle puts an opponent to sleep, it's accuracy is low but statistically you should land it within two or three turns, Meganium is bulky enough to take hits during that time and due to Sleep Clause you only need to land it once usually. Rest is a recovery move that can be used instead of Synthesis, it could work quite nicely with Sleep Talk as well. Toxic is a helpful filler move but it doesn't land against Steel and Poison types though who both wall Meganium pretty effectively. Hidden Power [Ice] is probably the most effective Hidden Power Meganium can use, it hits Flying types and Grass types for super-effective damage as well as the lesser seen Dragons, it'll also make the common Gligar regret switching in and trying to set-up against it. HP Fire is also effective, it hits Steel, Bug and Grass types for super-effective damage and with Sunny Day being fairly common in UU it can gain a handy power boost as well.

Strategy Against Meganium

Unless Meganium is carrying a super-effective Hidden Power a large majority of Grass types do well against it, they are immune to Leech Seed and resist Grass Attacks. Grass/Poison types in particular do well since they have a physical STAB option to use against it and are immune to the lesser seen Toxic. Steel, Poison, Bug, Flying and Fire types all resist Grass attacks and four of those have super-effective STAB attacks they can use against Meganium, if they have Substitute they don't need to worry about Leech Seed either. A lot of Pokémon can set-up against Meganium, Tentacruel, Scyther and Gligar are some good examples, all of which have super-effective moves and the potential to use Sword Dance. Over-all Meganium's offence is weak so if you can set-up a Substitute to block Leech Seed you can set-up against it if you don't have a weakness to Grass Attacks.

Contest Moveset

A good Contest Moveset for Meganium would have to be this for the Cool Contest best with Lonely, Adament, Naughty or Brave Nature:

Attacks:

Leech Seed
Sunny Day
Solar Beam
Hyper Beam

Items Attached:

Red Scarf

Strategy Using Meganium

If you do the attacks in this following order you should have very few problems; 1st - Leech seed, 2nd - Sunny day, 3rd - Solarbeam, 4th - Leech seed, 5th - Hyper beam

Locations in Games

Ruby/Sapphire/Emerald Trade from C/XD/E (RS) Evolve Bayleef (E) Colosseum/XD Evolve Bayleef Fire Red/Leaf Green Trade from C/XD/E

Animé Appearences

Meganium has appeared in several episodes. Twice owned by Casey, Twice owned by Jackson. Ash has battled Jackson and has befriended Meganium. Ash's Bulbasaur had a fierce battle with Meganium which ended in a draw. In The first special it was in, Jackson and his Meganium appeared to help stop Raikou being stolen by Atilla & Hun. After that, Officer Jenny used one to help against a Gulpin swarm. Later, Ash's Grovyle fell in love with one: Episode 263: Here's Lookin' At You Elekid Episode 270: Tie One On Special 03: Raikou! Legend Of Thunder Special 12: A Date With Delcatty Episode 341: Gulpin It Down Episode 438: Odd Pokémon Out