Make your way to the Gatehouse from the starting location. You’ll have quite a number of Hive and a Wizard to deal with. Approach the gate and deploy your Ghost, though it will take him some time to open the gate. You’ll have to contend with Thrall and Knights off the balcony. A well thrown grenade and a Cursed Thrall explosion should wipe most of them out but the Cleaver Knights aren’t so weak so use a Hand Cannon or Shotgun to eliminate them easily. The second wave brings Acolytes into the mix and it starts to get messy at this point.
Stay spread out to cover all angles and focus on the Witches then the Cleaver Knights. If a teammate falls be sure to Revive him quickly because the final wave brings a Major Wizard. Use Super Abilities and Heavy Weapons if you need to clear these guys out because you’ll have plenty of ammo to collect once the area is secure. Follow the cave down and you’ll be met with an Ogre and a Major Knight, though Thrall and Acolytes join the fray too. Focus on the lesser Hive before focusing on the Knight and Ogre; there isn’t a whole lot of cover so keep moving.
The Summoning Pits
Head to the left and defeat the two Major Knights that guard the entrance to the Summoning Pits. Phogoth , the Unchained is an Ultra Ogre but during this battle he is likely the least of your worries. Phogoth uses an Eye Blast constantly and it deals quite a bit of damage though ducking behind cover makes it easy to avoid. Stick to the upper levels and defeat the Hive that attack together as a team before focusing on Phogoth as the number of Hive will rapidly increase and could lead to your Fireteam being overwhelmed.
If you get too close to Phogoth he will emit a groundshaking slam on the ground knocking you back, so keep your distance. His weak spot is his chest and it can be difficult to hit at times because Phogoth slouches often. Use all your most powerful weapons on Phogoth and only Revive teammates if you can do so safely, especially if you are the last one standing. Do not leave a solid position to revive someone who is in the middle of a swarm of enemies.