If it's in between low-level conifers, you might have to paint the leaves of the mint with weedkiller. I did this last year by mixing glyphosate (Roundup or similar) with wallpaper paste and painting it onto creeping buttercup that was infesting some low-growing conifers and heathers. It was a backbreaking job but it worked really well - no buttercup this year. Roundup now comes as an expensive gel too, so that's an option, but I found a paintbrush was useful for small leaves.
