If you, like me, are having problems getting this Achievement, I think I've figured out the best way to go about it:
1. Try and enter Wintergrasp at a time of day when the opposing faction has as few stacks of Tenacity as possible. If you're on Proudmoore-US where Oceanic Horde are almost non-existant, this means do it when the Yanks are still awake, preferably on the weekend.
2. Enter Wintergrasp before the battle starts and get ready to claim a cannon. Don't bother entering a tower beforehand, as you'll likely get ported out. However, do mount up - you'll have a few seconds before you get knocked off your flying mount to fly into position.
3. Try to get a fairly central cannon - I usually like either the middle guns (if Horde are likely to get a couple of walls down) or one of the tower ones.
4. Even if there is absolutely no action on your side of the keep, don't give up your cannon. Yes, this may mean you get stuck with less honour, zero HKs and therefore less than 3 tokens for a win at the end, but if you give up your gun someone else can and will take it.
5. Try and predict where your target will move to when you fire. This is especially important when they're far away (and if you have the dreaded Oceanic latency) - if you fire directly at them, they'll have moved on.
6. Fire randomly into crowds, at vehicles, at popular paths and at the workshops whether you can see any mounted players there or not - you might get lucky.