Need help for spell

Hello,me and my friend started make a map and we need a spell help so if someone tell me how can i do it.

-Its a spell upgrade damage temporary for every ennemy unit in a AOE of 500

at level 1 its give 0.5 dmg more dmg per enemy unit,level 2 give 1.00 dmg per enemy,ect.. there 5 level can someone tell me how do this spell plz. and thanks in advance for time spend to help me.

If you ask vexorian nicely in his "new" thread, then he might make it for u.