>Item: Iron Plate/Metal Coat
Raises power of Meteor Mash, your main STAB move

>Hone Claws
Important to raise the accuracy of Meteor Mash and make it actually spammable

>Bullet Punch
Gets rid of faster threats so you can go ahead and spam Meteor Mash easier

>Earthquake
Useful against types like Fire and Steel which either threaten or hinder Megagross's ability to spam Meteor Mash

>Zen Headbutt
Strong secondary STAB in case Meteor Mash does not do the job