You really need to have at least 2 healers in your team in order to keep the team alive since you don't restore health between battles.
My current set up is Luminara (L), Sid, Jedi Consular, as the core with the last two spots alternating between Dooku, Asajj, IG-88, or whoever I have available at max level depending on how many characters I lose in fights.
Going healer heavy has allowed me to completely clear the GW board for the last 4 days (I'm level 60). Where as before (global launch) I could only clear up to 9 of the 12 battles if I was lucky because AoE damage was insanely high back then.
You have to decide who your high priority targets will be for each fight.
If there's only one healer on the enemy team I'll always take them out first, the exception here for me is if that healer is Barriss Offee. This is because she's actually tanky as hell and if you can't stun/block her heals she'll easily undo all the damage you do to her and you've wasted all your cooldowns while taking damage from the rest of the team.
If Sid is there I'll take him out next as I don't want him to debuff any of my characters with healing immunity. Some people will rather take out Dooku next because he can be very dangerous with his multi attack potential.
Other than that you just need to keep working on your main characters and make sure you level up their gear and skill ranks as high as you are able to in order to get them stronger.
