Sp is hardly a problem for higher levels
ex 85+19 MND and my holy pillar 5, holy light 5 and rolling thunder 5 all have around base 90 sp cost (40-60 if you count my secondary WIS status points)
The problem with being an attack healer (at higher levels) is the fact theres hardly any aoes and too many staves that will Healing skills>attack skills, the pure dmg output is fine
An example is Urev(67), who hits roughly 20k with a lv 6 soul strike, however the skill is single target and has at best 1m range
Both of our rolling thunders hit 3-4k a tick for 5 ticks 4m AOE but we are practically sitting ducks
Chase Blister is overall the best skill, but the range of the homing is very small, and requires the user to stand still to do all the casts which pretty much means your risking close quarters
The best advice I can give to any healer is to stick with the heals and buffs until your lucky enough to come across 4-7 rank skills, because your attacks wont do much and soloin takes considerably longer than a party
but the good news is your dmg will still be near everyone elses if you build properly ^^