I think there's a flaw in how we handle dispelling, namely in that we try at all. Any dispelling attempt, no matter how successful, won't actually cancel the power. Even if you exceed the spell roll, the best you can achieve is diminishing the strength of the power, not completely dissipating it.
Instead of dispelling, I propose that we instead try counter-spelling. The difference between dispelling and counter-spelling is simple. Whereas dispelling is suppressing the enemy's spell, counter-spelling is attacking the enemy spell with a spell of your own.
"Dispel" a fireball and the fireball is there, it's just less powerful. Counter-spelling Is chucking a fireball at the enemy fireball. With dispelling, the enemy fireball does less damage to you, but it still does damage. With counter-spelling, your fireball explodes the enemy fireball before it hits you, completely negating any damage.
The reason I think we should stop dispelling and start counter-spelling is to reduce the damage we take from the enemy. If they fire a Kamehameha at us and roll a 200 while we roll a 300 for defence, we should fire our own Kamehameha to completely overpower it and do damage to them, instead of merely reduce the damage we take from the enemy Kamehameha by 50%.
EDIT: Look at it another way. Which is best in service to the Rule of Cool, countering a titanic glowing avatar of Gork by reducing its size by a third, or countering the avatar by making it fight a thermonuclear battle tiger?