In-game, almost any set can work, as the in-game difficulty of the AI is not very high.
Competitively, Reflect and Light Screen are great moves for Meganium, as it's usual role is being a Dual Screener, setting up both Reflect and Light Screen to help teammates. A good item to go with this is the Light Clay, extending the duration of the Screens from 5 turns each to 8 turns each.
For the other moveslots, Solar Beam is a definite no. Only in gen 5 did a method of inducing auto-Sun (excluding Groudon) come through, in Ninetails. Outside of sun, Solar beam takes 2 turns to charge, allowing the opponent to switch into either a resist or set up on you. Furthermore, 2 Giga Drains takes the same amount of time and deals the same amount of damage, as well as healing Meganium. For the last slot, I would recommend Leech Seed, to enable both passive Recovery and slowly damage the opponent.
The final moveset should look something like this:
Meganium @ Light Clay
EVs: 248 HP / 252 Def / 8 SpD OR 248 HP / 8 Def / 252 SpD
- Light Screen
- Giga Drain
- Leech Seed