Meganium! oh man this thing is like the black sheep of the starters in all the generations combined! props for being a fan and user of this thing
Okay the down side is that Meganium is too well rounded for its own good hence its rather low usage but it can be rather effective as a balanced tanky Pokémon, so lets roll with that, we'll also be focusing on its special attack stat so its not just some stalling set-up fodder, Given Gen 6 has a stronger Physical meta at the moment I'd recommend a
Bold nature though
Calm is also good if you prefer a slightly more specially defensive nature, either way your attack stat is taking a hit, As with many tanks
Leftovers will be your item of choice for HP recovery.
Toxic is great on just about any tank.
Safeguard Is also a great tanking ability and also helps to avoid status effects, which works will with...
Petal Dance! Its one of the most powerful grass type moves in the game and
Safeguard should protect you from the confusion when its over! if however you don't like this 2 move combo you can also look at
Energy Ball as a solid alternative.
Also to add even more tankiness go ahead and add either
Leech Seed if you intend on having a
Raindance tactic in your team, otherwise
Synthesis (it only heals 1/4 of your HP in the rain compaired to 1/2 in normal weather),
HP for days!
For the above set you'll want a IV set of all maxed other then the attack stat and for your EV's you have a lot of options available, but 252HP would be the best, after that you can either split your remaining points equally between you 2 defensive stats, piles them all into a single defence stat to make Meganium a more specialised tank or even boost its special attack to let your STAB attack bring the pain.
Meganium can also make for a great supporter as well with
Aromatherapy, Light Screen, Reflect and can even use
Giga Drain (if your passing up
Synthesis) if you want your opponent to foam at the mouth.
I couldn't go over every single option she has as with those balanced stats you have a multitude of options available, its just you'll find other Pokémon that can do it better.