You can do bascially what you'd do with normal magic.
You just can't kill someone directly. Them doing something towards you, like momentarily setting you on fire or such, would give you the advantage to strike back, despite how powerful.
It doesn't matter how powerful of a spell you get them with, as long as they can at least try to strike back once, your castings will not bounce back.
Ex.
You get your opponent in a harmed state, after a while of casting back and forth.
Your opponent attempts to cast a water spell, it encasing you in water, but not for very long, as they are weak.
You cast a final spell, killing your opponent.
That would be fine. They had a chance, a few actually. They didn't use them well.
The thing that would be 'best' for Magick users, is that if the person that they are fighting against or harming is not a magick user, then it does not bounce back, but just makes you feel ill for a few hours.
Note-
Simple things, like changing peoples emotions, or making them feel sick, or dizzy, ect, is fine. It is not the same as actually harming them physically. Socially, Mentally, and Emotionally is not under control of The Darkness, though you still harves tthe power from it.