This cheevo IS NOT missable! At least if you are willing to buy the Sequence 12 DLC.
The DLC contains the first sequence that, according to Desmond's counterparts in the real world, is inaccessible at the end of the game. It continues the story with additional quests. After the opening Sequence 12 quest, where you must protect a couple people as they return home as their city is under siege, you once again have free-reign to go where you want.
If you travel to the NE corner of the map (around the marsh-like village) you can climb the light-house and play what the game calls a "Special Memory". Special Memories are indicated on the mini-map with an Assassin Symbol against a white bordered diamond.
This particular Special Memory allows you to fly Leo's flying machine whenever you want, and yes, you can still get the cheevo at this point.
Once you climb the tower and walk into the highlighted quest marker you will go to a short cut-scene of Ezio donning the flying machine. As soon as you have control of Ezio again you can spot a tower connected to a house pretty much dead ahead, and although you cannot see him right away, there is a guard standing atop this tower. This is a perfect opportunity to get the cheevo as there is nothing to run into on the sides, and if you miss you can easily fly to a fire to get altitude and turn around and try again.
As you approach the tower try to level out while aiming for just above the peak of the tower. Flying level slows Ezio's speed, giving you extra precious seconds to react. Once you are close enough to the guard for him to be highlighted, tap the left trigger to lock-on to him, then keep your course level and aimed for him. If you hit the guard and knock him off without kicking him you can fly away to a bonfire, and by the time you return he will most likely have respawned.
Although originally this cheevo was missable (and I did miss it), if you can/want to download the Sequence 12 DLC you can still get it. I love the Creed games, and think both DLCs are so worth the small amount of MSP.