There are a few ways to do something like this.
I presume you are wanting to hold a button of some sort, so you can do something like:
Player holds button
------ Set globalvariable.playerdamage to globalvariable.playerdamage + 1 * dt
Player releases button
------ Set globalvariable.projectiledamage to globalvariable.playerdamage
Projectile collides with enemy
------ Minus (globalvariable.playerdamage / 10) from variable.enemyhealth
By adding 1 each tick (using DeltaTime (*dt)) will give you a large number after a few seconds. You can divide this my 10 or 100 or whatver amount suits your damage scale. You can limit the maximum charge amount by adding extra conditions to the "player is holding button" event, such as "globalvariable.playerdamage is less than 1000".
Hope this helps.