Disintegrate is a cool spell I'm just not sure about my implementation.
Consider these changes for implementation.
Mana cost is 2X.
X = Level of Target Creature. Target creature receives X critical damage from this Attack.
In this case you would not have an attack bar since you are not rolling dice and the attack will always deal X damage to the target creature.
If you did want to include the attack bar so that a variable amount of damage happens, then you could change the text in the red box to 2X and keep the Critical Damage trait. Thus, attacking a level 4 creature would require 8 mana and roll 8 dice worth of critical damage.