Just a note: it is NOT a figure eight (infinity sign). I tried that for hours with no success. As soon as I tried the upright hourglass, it worked. Don't get confused about this. Also, make sure that you are starting at the upper-left and going to the right. As mentioned before, it's like drawing a 'Z' and connecting the ends.
To me, the hardest part was stopping it at the right time. If you're in the first phase of the Final Boss, Bellum will be flying around in circles, opening and closing his eye at random. My best advice would be to stand on the platform where the stairs are (he can't hit you there) and watch him go around. Wait until he's near you and his eye is open, then quickly draw the hourglass to stop time. Run up to him and slash away at his eye. Repeat.
If you're in the third phase, watch the top screen and wait until his eye is open, then stop time. Quite a bit easier, I think.
Hope this helped.