Attacks with STAB and subweapons from high Sp.Atk while a stall + special attacker build using recovery moves. Thanks to its ability, Solar Beamit can use in one turn, Synthesisand Synthesis recovery is boosted to 2/3 max HP, which is powerful.

Alternative Move Options

STAB Dazzling Gleam is also an option

Teaching it STAB Dazzling Gleam lets it hit Dark, Fighting and Dragon types for super effective damage. In particular, Solar Beam and Weather Ball alone lack Dragon coverage, so it's recommended if you want to handle Dragons.

Leech Seed Stall Role

Leveraging teammate weather support and 143 base Sp.Atk, a role of pressuring the opponent's team. With recovery, it absorbs hits while sitting as an attacker.

Threats

Fragile to 4x weaknesses

Mega Meganium is 4x weak to Poison, risking OHKO when hit, Synthesisor recovery can't keep up. Beyond Poison-types themselves, Poison Jab or Gunk Shotwatch out for Pokémon likely to carry such subweapons.

Heavy Sp.Atk/Spe EVs, attacking while boosting Speed via Trailblazeto push high-power moves from above — Mega Meganium build. After flipping Speed order, push STAB special moves to pressure the board.

Alternative Move Options

Dazzling Gleam swap candidate

Dazzling GleamThis slot — usable super-effectively vs. Poison etc. Earth Poweris recommended. Both are strong — pick per team/meta.

Special Attacker Role

TrailblazeBoost Speed with it, and break opponents with Mega-added Fairy STAB as an ace. By raising own Speed, attack and pin opponents normally not outsped — its specialty.

Threats

Fragile to 4x weaknesses

Mega Meganium is 4x weak to Poison, risking OHKO when hit, Synthesisrecovery can't keep up. Besides Poison types, Poison Jab or Gunk ShotWatch out for Pokémon likely to carry such subweapons.

Invests EVs in HP and Defense or Sp. Def, LeftoversMeganium build that raises long-game survivability via this item.Leech Seed and Substitute, Protect combines to buy turns, and Gradually chips opponents with fixed damage and recovery — its composition.

Role of the Leech Seed / Substitute Endurance build

Leech Seed drains HP from opponents, and Substitute and Protect uses skillfully to safely heal and buy time as its role. Weathering opposing attacks, Keeps own HP high with Giga Drain while finishing off opponents — core gameplay.

Special attacker build that invests heavily in HP and Sp. Atk and attacks aggressively with the Drought (Mega Solar) Ability Mega Meganium. It leverages its added Fairy-typing and high Sp. Atk to use the STAB AoE attack Dazzling Gleam spams to pressure the opponent's board.

Alternative Move Options

Protect can be replaced with Synthesis

Protect instead, Synthesis can also be adopted. With the Drought (Mega Solar) Ability, Synthesis's Heal amount becomes 2/3 of max HP increases.

Role of the Doubles-oriented Modest HP/Sp. Atk build

Solar Beam without charging and Weather Ball converted to Fire-type — plays the ace role of chipping a wide range of opponents. Leverages high bulk to stay on the field, while focused attacks from the opponent are Protect, blocking with it to maintain an advantageous situation — a flexible playstyle.

Threats

Struggles against Fire-type Pokémon

Struggles against Fire-type Pokémon. STAB Grass and Fairy moves, Mega Sol becomes a Fire-type moveWeather Ball — everything is halved. Water-types favorable against Fire and the Ability Drizzle Pokémon should be adopted as a counter.