you want the combo to start as soon as you press a. that means once you get to flints "attacked!" box, hit a as the beginning of the combo
also practice is a whole lot easier once you get to osohe castle and pick up the battle memory book from the present on the first floor. it lets you battle any enemy youve encountered before for as long as you want. you dont take any damage or anything, its there specifically so you can practice the various music. really neat addition i'd say