The rolling your arms like a disco dancer on a Speed Punch Bag and lowering your aim to body or head by squatting or rising from a squat works well to get the opponents Health down, as does blocking until you get the 3 Silver Gloves needed for a power punch.
The uppercut required for the Achievement is not a Power Punch though.
Just get your opponent down to the last chunk of red then Upper Cut...
How to do an Upper Cut :
I found that an exaggerated fist from belt to vertical,
(like you want to be first to be picked by teacher),
worked for me...doing a "proper" Upper Cut only registered as a body blow...Boooooo !
You'll know you've got an Upper Cut when you hear a high pitched SMACK sound...as opposed to a dull thump..get some practice ones in as you work towards success.
Best of luck Ladies and Gents.