target switch often, and try to kill the hunters pet. make sure you sunder the target you're going for, and if you can kill the pet, you might be able to get your druid a chance to drink (although drinking seems less frequent in modern pvp arenas). the point of target switching often is that you're making the mana not be mana efficient. everytime he recasts earth shield after just 1 charge being consumed he's wasting mana. likewise sundering your targets will help you deal more damage in the long run. be vary wary of the shaman, if he puts up earthshield on himself keep dps'ing him, he'll have to cast spells to keep himself up which will drain his mana, the moment he puts water shield on himself though you might need to change targets to make sure he doesn't gain mana from your attacks. and again, targetting the pet, if you kill it it's 1 less thing to worry about and it gives your druid more freedom. if he spams heals, you'll drain his mana quickly.
it depends on how much arp you can get and just in general what you're comfortable with. AJ apparently reports that ArP will always yield more damage even against clothies but definitely more against plate/mail wearers (in this case mail for you). It really doesn't work out as well though if you don't have an ArP proc trinket like grim toll/mjolnir's runestone.