This is ONLY achievable during the mission 'Infrequent Flyer' near the end of Sequence 8.
This trophy is missable as it is only attainable during the mission 'Infrequent Flyer' which occurs near the end of Sequence 8 after you first test the machine for Leonardo.
In order to kick a guard you will need to firstly lock onto them with
and then get close to them to automatically kick them off, the game will explain to you how to control the Flying Machine.
If you fly straight after the second fire bowl there will be 4 guards waiting to be kicked off for you, heres a video, credit to TrophyCollectors