Attack Deathbird any way you can at first... If she is flying, use ranged attacks. If she swoops down to grab a hero, follow the button press prompts to keep her grounded for a while, where you can melee attach her.
Keep this up until she is down to 50% health and she will start to get a boost from the crystal in the center of the room. When this happens attack the crystal and it will break that bond so you can attack her directly again.
Repeat until you take her out.