It's been too long since I've done battlegrounds at those levels, but as mentioned before, don't bother caring about performance. You'll be one-shotted and corpse camped by level 19 and 29 twinks.
At those levels, however, I recommend the Imp, simply because it's extra damage. The Voidwalker's Sacrifice ability is a godsend, but you'll have to summon a new one afterwards and not only does that cost a lot of shards, it also takes too long and burns through your mana faster than a Mage on crack.
Your job is to kill the enemy players. Now, I realize what you're asking, but know that there is no single role for a class in any battleground. You can't simply label yourself a 'defender' and go sit on the bench while the offensive players go to town in the midfield.
My advice is to find a healer, any healer, and stick to him like flies to a Tauren. Warlocks are nasty with their massive damage and relatively high survivability, but a Warlock with a healer is God's left hand. The hand he uses to ***** slap annoying people with.
Whether you run the flag, defend or go nuts in midfield depends a lot on the situation. Just note that you work best in a crowd (so you don't get singled out too easily) and that going toe-to-toe with a melee class at those levels is suicidal.
As for your approach to the battle, you should focus on staying behind the melee combatants and tossing DoTs on everything that moves. Everything. Hand those DoTs out like it was candy and don't stop until you go out of mana. This is where having a healing buddy makes it awesome. Simply Life Tap and watch those heals roll in along with your mana. Rinse and repeat.
Do I use Healthstones in battle? But of course, if I have any. I usually do a quick calculation on chance of survival first, though. There's no reason to waste a Healthstone if you're being jumped by four Rogues. It'll only delay the inevitable. However, if you're one-on-one and that Healthstone might make the difference, by all means, pop it.
I suggest you find a Holy Paladin and stick to him. Blessing of Protection, which the Paladin learns at level 10, makes you immune to physical damage for 6 seconds if the Paladin casts it on you.
That's six seconds of button mashing and tossing DoTs. Having no way to defend yourself against any class that can stun, you'll have to rely on outside help in order to increase your survivability. With Blessing of Protection, Cleanse (or Purify at those levels, I guess) and Blessing of Freedom, a Paladin would be my first pick if I could.
Don't leave home without one.
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Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.