npc_spells_entries

[trevius]

I believe there is a setting for Proc rate. Maybe you have to adjust that for the 2 heal spells you want to alternate. I think if the proc rate is set to 0, it won't cast it at all. But that field may only be for damage based spells. I haven't played around with it that much, but I have noticed the same issue you are talking about before.

[Congdar]

I changed the type of the celestial heal to SpellType_Buff and gave it a low priority and it worked the way I wanted with the celestial heal being cast and then the regular heal if there was a need for a booster, but since the npc was casting the celestial heal every time it wore off, it very quickly ran out of mana :(
The proc rate was above 0 but it didn't seem to matter.