The opponents scratches don't count.It’s pretty much an impossible scenario. You can’t have a BIH and only one ball left on the table both at the same time.
If you mean that your opponent pocketed the last ball into a neutral pocket and left the CB in the kitchen, I’d normally use top spin only (maybe with a touch of outside) but depends on the angle he left you with.
btw - pocketing the last ball into a neutral pocket and leaving the CB uptable is a very silly thing to do in one pocket
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