Soulsoother will cure based on the amount of HP needed to be cured. If the amount needed is under 200 you'll get a Cure IV. If the amount is under 600 I believe you'll get a Cure V and any other instance is a Cure VI. I could be wrong on that last hp amount, it's been a while since I played it. If the current highest tier available is on a recast timer it will use the next highest instead.

Other than that I believe you're out of luck. Toss on Mana Conserver and a Dark Maneuver to help the MP costs when you can is my only advice.