Heal Bomb until TU. Yes, SP is going to be an issue for a while (until Magnificat as Priest), but be patient. It gets better. Hopefully you level up relatively quickly so you get your SP back. Another way to conserve SP is to figure out how many heals it takes to kill an enemy and change the heal level (with the mouse wheel if you have one) to a lower level to finish it off when it's low on HP. I remember it taking 3 Heal 10s and 1 Heal 3 to kill Firelocks at some point in my adventuring.
Holy Light is used for small, annoying things that can't be hurt with Heal/TU, at least until Soul Linkers come. I'm one that likes having all options available to himself.
Also, something I forgot to mention about Firelocks: They cannot hit you through Pneuma with their regular attacks BUT THEY CAN hit you with skills if they are within 4 squares of you EVEN IF you have Pneuma up. Always try to stay 5 or more squares away, even with Pneuma, just in case.