I forgot about Serperior's hidden ability. I can only imagine what a Mega Grass/Dragon form with Leaf Storm and Draco Meteor. Okay, I change my mind. That's the new set.
Assault Vest could help, but Tangrowth needs its set-up moves such as Leech Seed, Synthesis, and the various powders. Plus, it needs the EVs in defense if it's going to tank those super-effective hits. Tangrowth doesn't need a huge special defense boost, just enough so that it doesn't fall so easily to special hits.