Personally I wouldn't use a dice or number based combat system in this, mainly for the reason that the range of powers and abilities we could conceivably have are are to wide and varied to be quantified by a simple number, if everyone uses damage dealing spirit abilities then sure it will work but when you use something a bit more abstract to confuse or disorient an opponent to gain the upper hand it's hard to numerically compare that kind of ability with one purely based of damage.