Rango

I found that the easiest place to get this achievement was at the very start of The Return of Rattlesnake Jake.There is a jack in a box right at the very start of the level. Smash it and check that it gives you the Launcher power up (the power up you get is random). If you don't get the power up you want just smash the three sets of shot gun shells which are nearby to blow yourself up so you can quickly die and re-spawn in the area having skipped the cut scene!
Once you have the power up you want just shoot all the enemies until the achievement pops. It is easier if you use the strafe mode (left trigger) as this allows you to keep your cross hairs targeted on the enemies instead of constantly turning round. I should also note that I had fully upgraded my launcher in the shop before attempting this achievement.
This solution will also work for the spreadshot and blaster achievements so you might want to go for them all at once by just dying and re-spawning into this area.