Rock of Ages

This may seem difficult but can be quite easy and are 2 methods to do this.

Single player:
What you need to do is create a wall of towers across the area in a place on the path where there is no way to avoid them, if possible try to make it 2 rows across and add some catapults behind to give some extra bump to your enemies boulder. If you have some spare cash and can also afford an Elephant place it in front of the wall so it will help bump if off and give you more time to hit it.
Do not launch your rock stay waiting for it at your new wall of towers with the Castlepult equipped. The catapults should slow it down before it hits your towers so it shouldn't barge through that easy. Here it should become trapped, hopefully within a block a set of towers, from here just shoot and hope it hits, you may need a few shots but it is easier than trying to hit it while it is flying down the path.
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Multiplayer:
The easy way to do this if you have a 2nd controller like most achievements in this game is to start a 2 player split screen game on and level. From here get the 2nd player (Player 1 DO NOT LAUNCH) to launch and then hold still or lock themselves against a set of stairs press 'RT' to open your menu and select the Target (Worth 0 points) and aim on them and press 'A'. If you hit them it is all yours.

This achievement can be boosted by playing online with a boosting partner. (Or in a local split-screen match - Thanks CyberellaD Nile).

Have your opponent launch their boulder and move it forwards a slight ways, then stop altogether. Press down on the D-Pad to center on their boulder, and then fire your castlepult.

(Castlepult is used by pulling RT to bring up the building menu, and then selecting the "target" icon.)