Earth Shaker is best as a stunner and initiator. If you can catch the enemy team off guard with his ultimate they will take a lot of damage. The problem is closing the gap. This can be done with a blink dagger. Otherwise you will be focused and disabled to no end attempting to walk into a team fight.

Scepter and refresher orb will practically enable him to wipe out the team alone. Make sure you have enough mana to do that combo first before going for those items.