you cant make 15 clicks per second normally to get that achievment? Dont worry ! theres another way to do it :D
1. Get an PC with 2 entrances for mouses ( you should already get the idea of what you have to do by now )
2. Get 2 mouses and then, connect the 2 mouses to your PC
3. It might be tricky at certain PCS that glitch a bit with 2 mouses but use only 1 mouse ( otherwise its kinda hard to ) to put the cursor at the big cookie
4. Do an quick test, First, use only 1 mouse to test how fast you click with it, and then the other one. So you can try to find out which mouse you can click faster with. Then use that mouse on your right hand, ( Or left hand, if you are left - handed ) And the other mouse at the other hand. So you will have the max of clicking speed you can by doing that.
5. Click as fast as you can
6. You might take a while to but you will most likely get the achievment like i did with this trick. Hope this helped you.