Basically having sustain offensive spells avoid cooldowns and essentially the spell limit so they can spam the high rank spells every post.
Maybe lower the damage one rank if it's offense sustained(Save D ranks since that's the lowest) or increase the mana cost so it's not abused by everyone as it's currently trending. The other option would be to delete cooldowns in general.
just to show how unbalanced it is here is an example. Let's say the user only has One A rank spell in their spell slots.
Normal
A rank normal damage
4 post CD
One time only
200mana
sustain
A rank normal damage
4 post CD(like it matters lol. you have it every post)
A rank damage per post only(what spell limit?)
200mana+200 mana per post(aka spam till OOM)