Bashira requires 3 silver archers to CE/CC and Soma requires only 1 archer. The story missions give you 4 silver archers in total from drop completion. So, like what anon above me said, do not focus on skill levelling your archers until you get an excess amount of them. And since you don't plan on doing any premium summons anytime soon, the chances of you getting a lot of silver archers is quite low from 2K summons. For levelling up healers, just use the standard iron and bronze units you get from Return to Ruins, Deep Forest Road, Pastoral Gate, and/or Gatehouse Battle. If you get past Castle Retake, do Dragon Hunting/Flame Dragon for grinding.
You should also aim for Spica the platinum archer in the trading post while she's available. She costs 200 demon crystals, which you could farm in Thursday dailies. After you get her, you can replace Soma for her.
Key lesson from experience: Do not feed silvers and above to your units. They are very important to CE and Awakening when it comes in March. In my main account, I fed my silver units to CR and regretted it since I was short on silver archers when the time came to CE higher rarity units like Bashira.
As for priorities on who to CE first, going by advices given to me in the past: Katie > Bashira > Cloris > Iris > Leeanne > Soma > Alissa > Valerie > second witch (Calliope I assume) > second mage > rest (valkyrie, Phyllis, another duelist and heavy armor)