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Ok let's move on
Meganium
>Old
>Hp: 80
>Atk: 82
>Def: 100
>SpA: 83
>SpD: 100
>Spe: 80
New
Hp: 120
>Atk: 100
>Def: 130
>SpA: 80
>SpD: 100
>Spe: 60
>New HA: Aroma veil
>New moves: Rage Powder, Solar Blade, Stealth Rock
Not sure how good it would really be but could potentially be a tangrowth rival though the lack of regenerator would hurt it.
I think Aromatic veil is a nice ability for it thematically and would allow it to set up screens and make use of synthesis and rage Powder without fear of taunt. The combination of Rage Powder and Aroma Veil would allow an allied Charizard Y to set up tailwind against non rock slide leads and it gets heal pulse to support a sweep.
Finally I decided to lower the speed and increase physical attack over special as Meganium gets some alright physical moves like Petal Blizzard, EQ and Dragon Tail.
600 BST is hard. I don't think Meganium needs more stats as much as it needs better typing and ability.