1: SL/SL is apparently the way to go for Arena (Don't do arena myself because of
latency)
This spec lets you outlast your opponent until (if caster) all their mana is gone or (if
melee) all their health is gone.
Demo locks (felguards) also do very well in arena. Aff is right out unless you can hide
in a big 5v5 team
For BG's Aff do a lot better but still are very squishy if you ever come face to face
with someone. Destro is also very good but extremely gear reliant. And Demo is King.
2: In BG's I always have my Felpup out (i'm aff) for destroying casters but if you have
Felguard use him. Succys are good for when a rogue gets the jump on you and you
can seduce them but I prefer my felpup as he is just so versatile (good mana pool for
DP, not bad dps, and tough. Not to mention spell lock and dispel magic

)
3: SL/SL - felpup
Demo - felguard or if you see they have a priest quick summon felpup
Aff - Felpup/succy your choice.
4: Get your dreadweave set. You want as much resilience and stam in PVP as you can
So you don't get 1 shotted by rogues.
5: Arena Gear
6: If you see him first send you pet to attack (even if he stealths you pet will still attack him) and dot him up (this will break stealth & keep him from stealthing) If you have Curse of exhaustion use that as well, although it does no damage it will slow him down making it harder to close the gap. Then try get a fear off, SB & reapply dots. Save your Coil for the end, you want to force him to use his cloak of shadow before you use it. If you have shadowburn don't be shy with it, an extra +1000 instant dmg does wonders, and hope to god for shadow trance procs.
If he gets sees you first and manages to stun lock you, then pray. While praying, if you have succy seduce him or if you have felguard intercept him to buy precious moments. Use trinket and DC or preferably iHoT him if you have it, just get him away from you. If you seduced him don't blow your DC, just create a gap drop a CoE on him if you have it. Then follow the above.
Maybe try fear him as an opening attack if he is seduced to let you dot him up without breaking cc.
And try to stay calm, don't panic waste your DC save it for near the end of the fight. Unless it's unavoidable.
Also if you suspect there is a rogue stealth in your general vicinity a blast or two of lvl 1 Hellfire is a nice surprise to force him out of stealth.