The Gravetide Summoner is a bounty that you can purchase from The Spider on the Tangled Shore. It will cost you five Ghost Fragments, and unlike other bounties, it will take the form of an Adventure. To start it, head to Titan, and spawn at Siren’s Watch. The totem you will need to interact with is right beside the spawn point, down at the entrance to the Methane Flush Lost Sector.
Interact with the totem, then head down the tunnels towards the Lost Sector. When you arrive, there will be all sorts of enemies killing each other, so murder everything and head deeper into the rooms. At the back, you will find the Gravetide Summoner.
The Gravetide Summoner is a Hive Wizard, and she will be accompanied by two more of her kind. Kill both of the lesser Wizards, as the Gravetide Summoner is invulnerable, and she will then summon in a giant Ogre. This is a fierce enemy, and it can take a tremendous amount of damage. Grenades Launchers, Rockets Launchers, and Sniper Rifles are all great weapons to use against it.
While you fight the Ogre, the Gravetide Summoner will continue to spawn in other enemies that you will need to deal with. These include Thralls, Knights, Wizards, and even Shriekers. You will need to clear them all, while also dealing with the Ogre. There is a raised platform at the back of the room that gives good cover and allows you easy access to both the room with the Gravetide Summoner and the surrounding rooms. It is the best place to operate from, as it gives you good angles of fire on the Ogre, and keeps you safe from the other enemies.
When the Ogre finally falls, you can now damage the Gravetide Summoner herself. The good news is that she is not particularly tough without her squad to back her up, so she should fall very quickly. When she is dead, the bounty will be finished, and you can hand it in from the Quests tab in your Director.