You will naturally unlock this achievement by doing all the other stuff (freeing all garrisons, doing every side quest, etc). However, if you want to speed things up AND work in another achievement which will reflect on this one as well, do this:
You can spawn INFINITE dragons to kill on the very same spot IF you do the following:
- During the "Go to the Dam" objective. After planting the explosives and their failing to blow up, you are told to go meet a civilian engineer. Once you're on the bridge, instead of advancing through it, jump off to the left side of the river, then swim all the way onwards and climb the rocks beside the fence where the objective marker is. Then go where the
dragon marker is. It's going to be there pretty much alone and waiting the regular turn of events. But instead of attracting it to fight your enemies on the bridge, you're just going to kill it and then go back through the same way you came from, swimming back to the beggining of the bridge. Once you get there, a new dragon will spawn on that same place you just came from, so go back, rinse and repeat.
That way you can farm dragons and get this achievement out of the way, so you won't have to worry if you'd find enough dragons nor if you're going to need to reset the garrisons once you free them all for that other achievement (this way you won't have to). Another advantage of this method is that you will end up
farming XP too (each dragon gives you a whopping
5000)
so you'll reach level 30 in no time, also unlocking this achievement.
Finally, just as a reminder, there are quite a few elite enemies on the bridge, so remember to kill them with headshots, so it also counts towards another achievement.
If this solution helps you in any way, give it a thumbs up. Cheers!