recommendation letter dilemma

I did not realize you need 2 teacher recs for RSI, so I just asked one teacher and one research mentor. I need to ask another teacher but it's so last minute and the letter is literally due in 4 days, which makes me certain the letter will be sub-par. I also don't really know this teacher too well. But I don't really have any other choice. Is there anything I can do to make it as easy as possible for this teacher?