What spec is this dk. Unholy ? At 4k reslience, stuff is really tough to kill. Even with 5 guys melee zerging, it still takes a while to kill. Especially DK/Rogue, those two can survive for a long time.

and gear does make a difference. Not the only factor, but it is one of the factors. We are not frost mage who runs around being unkillable in PVE gear. Melees need pvp gear. As for boxing skills,

It ultimately breaks down to 2 parts:

1. the setup and 2. awareness and reaction time

The 2nd one is hard to get. It takes a lot of practice, OR even talent. The first one (setup) should be achievable for everyone. i.e. For a case like 5 v 1, simply put all your defensive CD into your dps spam, even Hungering Cold round-robin, you will beat him. No way he can kill you if you used up all your cool downs. Since the stuff are all in your dps spam, even if you panic and only hit the dps key. It should still be enough to kill him.