Unfortunately you can't do this in practice mode by setting the health low.
You can however do this in pretty much every other normal mode.
Personally I did it with a second controller in local matches and played as Sabrewulf because his Ultra is very easy (backwards then forwards and three punch)
if you're using default controls.
If you don't have a second controller then the next easiest method would be CPU matches.
All online modes work too.
You can cancel ultras
to speed things up, you'll still get progress. Doing a double ultra by doing a instinct cancel will not get you double progress.
You get 4% per fight and you'll get it after it shows your characters victory ending.
This should take about 40 minutes at the most if you do it with local matches or bots.