Well first off you are really not abusing its "effectiveness" with its moveset. Draco Meteor coming of a base 60 SAtk will be doing nothing to absolutely everything. For your moveset I would run Fire Punch to deal with Ferroseed (or coverage if its not for an Ru team) in place of Crunch. Outrage in place of Draco Meteor (alternatively you could run substitute) and Earthquake over Revenge for better coverage.
So to answer the EVs part I would simply run 252 HP and 252 Atk. Though if this is an Ru team 20 EVs in speed is recommended to outpace Tangrowth. The Evs maximize its bulk and power as speed is not needed.
Druddigon @ Life Orb
Trait: Sheer Force
EVs: 20 Spd / 252 Atk / 236 HP
- Dragon Claw
- Fire Punch/Sucker Punch
Sheer Force is prefered over Rough Skin if you are to use Life Orb to loose its damage while power up some moves.
Edit now you have decided to change your item...
You will still be wanting a very similar spread to accompany your attacks and bulk. Though you could consider investing in Def though in general Rocky Helmet is very situational and may not be so useful, similar to how uncommon rocky helmet Ferrothorn in Ou.