As a general rule ranged will take defence buff, so enemy team will start with melee and maybe will send a tank to stun/double Fell Cleave you. What you need to do is Fluid Aura tank and Repose him, then focus on pre-healing melee, because otherwise it will be too late if you start casting when his HP drops. The burst lasts only a few seconds, then they'll do only moderate damage for a minute or two. It's OK to Tetra someone if his HP drops below 40%, but save Bene for yourself, you'll need it immediatelly after Attunement when they'll focus you (and they eventually will, usually after melee death). PvP skills like Sacred Prism are great, but because they have a long cooldown, use them only before or during enemy bursts.
No matter what, you'll lose a lot at start of your pvp career and will be insulted almost every match (till 3.5 at least when they'll mute everyone), but as a compensation you can enjoy instant pops one after another while they'll have to wait 20-30 mins for the next match. And if you'll imrove you'll rank up fast because these instant ques (nobody wants to heal in pvp because it's too hard and ungrateful).