“The Cabal ship is locked. Decimate their forces outside so they will open the doors to send reinforcements.”
“Weaken the Cabal” is an objective in the mission Enemy of my Enemy in Destiny: The Taken King
When you land on the Dreadnaught you need to break into the Cabal ship. Outside the ship you have a number of Cabal and the Hive fighting. You will have a number of Acolytes, Ogres and Knights. They are fighting the Legionaries and Phalanx. Fight your way through the Hive to reach the Cabal.
To get through you need to kill around 9 Cabal to get the door of the Cabal ship Dantalion Exodus VI. When you get close to 80 percent, head over to the door for the Dantalion. Use this position to clear out the remaining Cabal. This lets you quickly access the area when the door opens.
When the doors open, keep to the side and let the forces exit the ship. Slip in behind them.