You will stand here and wail on the dragon until it eventually OSRS gold.If you approach the dragon's backside from the Site of Grace, you can stand next to it and continuously attack it without it attacking you or any other enemy attacking you. 

Depending on your level and your weapon, this process will be time consuming. If you have a weapon that inflicts blood loss, use that to help speed up the process.

Once the dragon is defeated, it will die along with the smaller dragons that are on the other side of it, netting you a cool 80,000 Runes. Once the dragon cheap RS gold is killed, it will not respawn, nor will the smaller dragons.