This can be done by using the Bullseye bonus. You can buy one from the shop for 500 crystals, or if you're lucky you may have already been awarded one from playing through the single player story.
The bullseye bonus deals 10% damage to the enemy when used. Make sure you equip it in your inventory before you begin the match. Once you get your enemy down to under 10% health, touch the icon to activate it and you should defeat them using bonus and your trophy should pop up straight away.
I found this easy to do during the story mode, but I assume you could do this online too if you wanted.
*Edit*
An update was released that has reduced the cost of all bonuses by 33%, so now the Bullseye bonus will only cost 330 crystals instead of 500.