Active skill damage question

How to make an offensive skill damage factor on hero's normal damage?
Say a hero cast a offensive skill which deal 200% damage base on his normal damage.
I found someone has done this in his map and as long as hero keep upgrading hero's weapon, hero's offensive skill damage will also increase too.

I find this is particular useful in rpg making, but have no knowledge how to start it off.

critical attack

You mean like if A Hero has 10 damage and a item gives him +10 damage, then do double spell damage?

I have no idea that doesn't involve just about a million triggers

critical strike is a passive skill. I would like a skill that can only be activated by players, much like diablo's offensive skills. Stormbolt for example is an offensive skill which deal fixed damage. I'm thinking to make attack skill deal damage % factor on hero's damage.

Try putting a units damage in a variable then adding it to itself, then make it deal the vars amount in damage? it im not sure if there is a way to do get units damage Welll try to look just incase