Thinderella (25G)
Destroy 100 enemies
I found that the best place to earn this is the Mad Doctor's Lab, while you are fighting against the Blotworx Dragon.
At the third stage of this fight you must move up an exposed path. On the top, three Blotling generators will continuously spawn Blotlings, and all you have to do is to kick them off the ledge, into the Thinner. One kick - one kill.
The Dragon will continue to attack, but it means no real danger.
You will have two achievements in a few minutes: "Junker (10G) - Destroy 50 enemies", and this one.