Find a way into the Tower and repel the attack.
Head over to the left from where you touch down inside the Tower. Head across the room.
When you approach the door on the far side of the room, it will be blown away. This will reveal Cabal Soldiers, Legionaries. Defeat them and head into the blasted open doorway.
Follow the corridor over to the left. You will see a frame firing on a Cabal Legionary. It will be destroyed before you can stop the Legionary.
Two other Legionaries will join the first and assault the Guardian. Quickly take them out using a grenade or careful fire on their heads.
Follow the corridor down to the right then left and deal with the Legionary who is in front of a door. Kill the legionary to trigger the door opening.