Need Help From Skilled PvP Hunters (Arena 2v2)

Hello there Hunters, I have created a hunter about 2 months ago and I like it, I would like to focus now more on PvP (Arenas 2v2), but I have that problem I dont know who to kill, who to focus, when to switch targets, etc...

What to do against these teams:

1. Hpala/Awarr.
2. Rogue/Mage.
3. Rogue/FeralDruid.
4. EleShaman/DestoLock.
5. ShadowPriest/Rogue.
6. ShadowPriest/Mage.
7. DiscPriest/Rogue.

I'm noob at arena, when I watch some videos people always start to kill DPS instead of healers, why?

I hope someone can explain me because I really want to learn and improve.

One more question, who will be the best arena partner for MM hunter in 2v2? I'm always playing MM Hunter.

Thank you.