After inputting the "Don Don Swing" you can continuously input 360's to make the attack stronger, that's what they mean by "Donvalve's strongest swing." Donvalve will rise higher and higher depending on how fast you do the 360's, so it should be easy to see how your inputs are being used and if you need to do more. When you've reached the highest and strongest version of the "Don Don Swing" a white flash will come from Donvalve's hand that's spinning the opponent, which he follows up by throwing the opponent downwards. You can do this achievement pretty fast as it has no other requirements so just put the difficulty on the easiest and you can do it round 1.