Crowd control, crowd control and again crowd control. Prevent the death knights from striking. Lower resistances, willpower, body building. Stun, freeze, slow, knock down, blind. Even if all of your party do nothing else for the first 2 turns all is good.
Forget damage. First, second and third - disable. Then damage without retaliation.
I have more or less the same party set ups (with a ranger or a battlemage instead of a second warrior) and found the death knight fights quite easy.
Regards,
Thorsten