The way I did this was to make a local multiplayer match with 4 players all assigned to one controller. Make sure the match type is set to Full Wormage for near enough infinite ammo for everything.
When the game starts, find or make a hole and use the Teleport device to place every worm on the map in the hole. Then simply throw a banana bomb downwards so that it hits everyone. If done correctly you should be scoring quite a bit of points (100 for every worm to be exact).
If you do this straight away then chances are you will get the "Bronze Damage" and maybe the "Silver Damage" achievements plus the "Three Bagger" one too. But for Gold just keep doing this same tactic until you get it.
Alternatively you could just play the game as normal and one day it will appear.
(PS this is a cumulative achievement, meaning that all the points are added together from every match you play - regardless of whether it is on xbox live, locally, against the cpu or against another human.)