The Pallas Siegebreaker is a Bounty Wanted that you can collect from Spider on the Tangled Shore. He's one of the enemies who escaped from the Prisoner of the Ancients, and it's up to you to hunt him down and take him down and end his regained freedom. The only clue you are given about the Bounty when you pick it up from Spider is that it can be found on the Tangled Shore.
Finding the Pallas Siegebreaker is surprisingly easy, and you've probably met it before, and you just haven't realized it. The Pallas Siegebreaker can actually be found during the public event which takes place very near the spider's lair on the Tangle Coast.
The Cryopod public event is all about taking on a few waves of enemies, then a big boss will come out of the Cryopod and you will have to defeat them. On the Tangled Shore, this boss is Pallas' siegebreaker.
To complete the bounty, you don't need to make the event heroic and can kill the siegebreaker. This event relates to two different quests, however, so if there are other Guardians there, it is considered good manners to ask the local chant that anyone needs to make the event heroic before doing so. melt the boss.
If anyone needs to make it heroic, you simply take a third of the Siegebreaker's health, then shoot the leaky vents on the side of the cryopod to expose a few orbs. Throw them to the siegebreaker, then repeat the process one more time to freeze it in place.
After that you have to defend the area from some waves of enemies and the siegebreaker will be brought back to prison. You can then return the bounty from your Quests tab in the manager.
