The Pallas Siegebreaker is a Wanted Bounty you can collect from Spider on the Tangled Shore. He's one of the enemies that escaped from the Prisoner of the Ancients, and it's up to you to track him down and take him down and end his newfound freedom. The only clue you are given about the Bounty when you collect it from Spider is that it can be found on the Tangled Shore.
Finding the Pallas Siegebreaker is surprisingly easy, and you've probably come across it before, and just didn't realize it. The Pallas Siegebreaker can actually be found during the public event that takes place very close to 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 pop out of the Cryopod and you'll need 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 is tied to two different quests, however, so if there are other Guardians there, it's considered good manners to ask the local chant whoever needs to make the event heroic before doing melt the boss.
If someone needs to make it heroic, you just 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 at 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 siege breaker will be taken back to prison. You can then turn in the bounty from your Quests tab in the Director.
