White Oak Stake - The only physical weapon that can permanently kill Sandy, It comes from an Ancient White Oak tree during the time of the genesis of The Originals.
Uninvited Invitation - In order to enter any house owned by humans, they had to be invited by that person. Once invited the vampire cannot be uninvited. If a vampire lives in a house, other vampires can enter freely and if vampire is in the house without being invited in it will become disoriented and will begin to suffocate.
Esther - Her knowledge of the spell that created vampires and thus ability to reverse it could be the deciding factor in this.
Magic - As a supernatural entity Sandy can be harmed or killed by a witch/warlock that can channel enough power. Considering his level of power however this would likely be very difficult to achieve.
Desiccation - Like any vampire lack of blood will immobilize Sandy This process can be artificially induced by a witch through the use of a dark magic spell which stops a human's heart and having other people make and hold a blood connection to Sandy.
Hunter's Curse - If a vampire kills a member of The Brotherhood of the Five then they will be tormented by the hunter until they kill themselves or if the hunter is replaced then they will be free of torment.