how to erase negative marks while settling a debt for less
It isn't possible. Creditors will update the account status as "paid as settled" or "paid as agreed". You should concentrate on adding positive items as much as possible. Your credit score will improve with time.
If you settle the debt it will be marked as paid for less than the full balance and will reflect a zero balance. That will increase your score. If you have an open trade line such as mortgage or car payment and are never late your score will improve dramatically. It will not be in the 800's but it can get up to 700. It is all about stabilizing your credit and showing responsibility after you settle