First of all, open up training mode and pick Gordeau, any map and opponent should do (I picked the one on the roof and Orie as my opponent). Then you want to immediately open your settings with the menu button (default being the "Enter" button) and set the exs gauge to default and all recovery options for the AI to off (rotate forwards and backwards through the pages with your two set buttons, default being "K" and "L"). Then you want to attack/push the AI to the edge of the arena and hit them until you have 40% of your exs gauge. Then you should hit the medium and heavy buttons at the exact same time (default being "I" and "O" respectively) to get your gauge to roughly 50% and then immediately go into the light autocombo (done using "U" by default), doing this three times in a row. By the end of this you should have enough to then immediately press the light attack button and the arrow key facing your opponent ("U" again) to use 100% of your gauge to get some more scripted hits in. That's it. Congratulations on your achievement!
*Fair warning though that you may need to practice a little because the final gauge attack that I mentioned can miss based on your timing - you must be precise!*