Getting this achievement can be tricky as most maps will quickly result in islands that you cannot reach. To prevent this, make a custom game against an skilled AI and play on the map Pioneer's Peninsula. It is very easy to just build walkers (and an armory or two) and out-maneuver your opponent. Once you have him locked in place you can capture the map and build as many cannons as you see fit. This achievement requires a few things before it pops, however:
1) A cannon cannot target any adjacent tiles, so you must have enough cannons to cover every tile in the game. Be sure to check the range of your cannon by pressing X
2) Your cannons must be armed at the beginning of your turn. Meaning, that if you have the cannons and they cover all the tiles, simply buying the ammo won't unlock the achievement. Every cannon you own must be armed and prepared to fire at the beginning of your turn. Remember you buy ammo one turn, but it isn't "available" until the next.
Get 1 and 2 and when your turn comes around the achievement should pop.