When you don't tank, you tend to think that tanking only consists of soaking damage and having a healer spam you at 100%I am far the best tank out there, having only played offspec blood for LFR and dungeons (up to about M+10). With 945 ilevel (and concordance 75) I managed to nail the challenge in about 2 hours. I had never even touched Vengeance DH and I nailed it last night with ilevel 940, and suboptimal legendaries. If I can do it, I have absolute faith that anyone with average tanking ability will manage. Just keep at it. It can be frustrating but you'll definitely improve the more you practice (and having done it on one class makes it easier with the second - I had to learn the spec and had worse gear on my DH and still manage in a similar amount of time).
To be fair, it does take a certain amount of tanking skills to nail this. Using damage reduction cds properly to survive the stacking debuff in P1 is how you're meant to up your dps. You need to interrupt stuff, you need to control a bunch of adds etc. In p2 it's all about the kiting + damage mitigation + add control + situational awareness. The "dps race" is more the way people with lower tanking skills handle it (like me :P)
It's actually what I ended up doing for Blood DK. Popped everything at the start and burned Variss down in 40 seconds using AMS and all my available interrupts to stop mind rend from killing me. Made P2 a lot more manageable because there were 0 infernals.