I know people might have figured this out/already gotten it by now, but it would be good for some who haven't been able to get it.
For the 'It's A Duck Blur' achievement, an easy place to get it is at the Amazon level, near the beginning. Instead of going down the first vine to the underground part, break the blocks to the right of the entrance and you should get the magic coin. Then, just run to the right, jumping over holes and walls, hitting as many enemies as you can. I managed to get kill six enemies by the time it ran out.