Many years ago Kevin P McAuliffe did a video tutorial on making a countdown in MC. The principle should still work today.

It can be viewed here. 

Gerry

I did it while ago. I used timecode generator, mixdowned it and reverse it. Then you can just crop minutes or frames etc. just to make it look as seconds only.

You can add it even to the real action background, but you have to reverse the motion of it, then add transparent TC and then mixdown and reverse the whole thing.

No need to reverse it - timecode generator has a 'remaining' option.

Unfortunately it's only sequence remaining, rather than clip remaining, so you do have to mix it down