>>7714
>ffmpeg -i "%~1" -c:v libx264 -pix_fmt yuv420p -threads 8 -ac 2 -movflags faststart -preset veryfast -c:a copy -crf 18 -b:v 0 "%~n1_264%~x1"

guessing "%~1" is your in file name, so this would be the format, though I'm not sure ffmpeg supports this.

ffmpeg -i "%~1" -c:v libx264 -pix_fmt yuv420p -threads 8 -ac 2 -movflags faststart -preset veryfast -c:a copy -crf 18 -b:v 0 "%~1"

BACK UP YOUR SOURCE FILE BEFORE RUNNING