I doubt you are not talking about your own website. You are talking about somebody else's website you want to damage ranking of. You are just hesitant to ask it straight.
Pls correct me if I am wrong.
If this is true, then my friend, this is a wrong and unethical business practice. I am sure nobody in this forum will support this idea.
I have a wonderful suggestion for you -
If you really want to down any websites ranking then build another website that is much better in terms of SEO and all and you will have out ranked that website. Problem solved.