Set your match settings to 30 seconds and one round and pick Zangief, then spam away on the Lariat. You have to beat each round with an *average* of 20 seconds, so don't worry if you get slightly over for a round or two while being well under for the rest.
If you're having trouble, try bumping up the difficulty from Medium - on lower difficulties the AI tends to not quickrise after getting knocked down and time is of the essence.