How do you get dr breen in half life 2

if you mean how do you kill him then basically what will happen is he will be floating up the column in the centre of the citadel, you should climb all the way to the top killing any combine forces that get in your way, don't spend too long doing this as you need to try and get to the top as fast as possible, once you reach the top and he stops moving, lots of metal will appear around him, you should equip the gravity gun and turn to your left, you will see a column with orbs floating up it, suck an orb with the gravity gun then blast it into where breen in stood, the key here is to make sure you domnt hit the metal with the orb, or it will simply bounce away, you need to be aiming for the bubble that breen is stood in, every time you score a sucessful hit you will see some metal come off, do this until he dies.

hope this helps :)