Getting Started: Combo Challenge runs are pretty straightforward. Timing starts when the last loading screen (the "Loading" in the bottom right) disappears before you gain control of your character, and ends on the final hit of the 10th challenge.

Tips: If you have an extra monitor. or even a pen and some paper, jot down the 10 combos for reference. This way you can start as early as possible. A lot of moves are animation-cancellable into supers and vanishes, like SS Goku's 214 M (QCB M (The spinning kick in his medium autocombo)). Doing this can save several seconds per cancel. Do note that some combos do not allow this, as they cause a whiff (Looking at you, Bardock). Also take note that certain supers have multiple hits, and the timing doesn't stop until the last attack of the list is golden. Lastly, if you start at the end of the combo challenge instead of the first one, it wraps around. If you are struggling with any combo or find one is just inconsistent, you can start on that one just fine.