Frost mages are killing machines in BGs, but at 27 you lack many of the key spells.
Keep your target at range with rank 1 fb. Spam ice lance. CoC if they get close, nova, blink, deep freeze and then a shatter combo. Pop out your elemental whenever you can and always keep ice barrier up. Ice block out of dots and fears.
There's the problem. You've got very few of those abilities. Basically all you can do is fb, nova and blink (I don't recall if you get CoC yet). Basically, stay on the run. Cast only instants like Arcane Explosion and fireblast unless you have room to get off a fb. Polymorph liberally. A shatter combo is usually done with a frostbolt and icelance but you can substitute a fireblast. Basically, when an enemy is frozen in a nova or by a frostbite proc, you cast a frostbolt and then the moment the frostbolt leaves your hand hit your instant (fireblast in your case). The two spells should hit at the same time and benefit from shatter, making them both highly likely to crit. It's insane burst damage.
PvP with a mage requires a lot more skill than with, say, plate-wearing classes with enormous burst and healing abilities. You're very fragile and it's a lot tougher to keep another player off you than a programmed mob. But it's doable, and a well-played frost mage can handle pretty much anyone after, say, lvl 40.