When you first come into a room with a Big Dog in it, you need to switch up your tactics a bit. Your main goal is to get his attention and make him reset.
How do you get his attention?
You run in front of him and if that doesn't work, use diagnose in front of him.
But how do you reset a monster?
It's pretty simple. All you have to do is run around the room. Pick two spots on the map with good 'hiding spots' and run back and forth between them until the Big Dog returns to it's original spot.
TIP: Run to one 'hiding spot', wait for the Big Dog to come almost all the way to you and them reposition yourself to your other 'hiding spot' and repeat until he resets.
Once you have the Big Dog under control you should have the aggro of the whole room, meaning it is time to call in your team. Make sure they know to save the Big Dog for last. A lot of Aggros will ask their team to leave one vamp so they can keep healing, but you do not need it. If you decide to let your team kill everything and then move on to the Big Dog make sure you toggle your rings and self heal yourself to keep the Big Dog's aggro.