Brotato

Certain items in the game have Negative effects for the positive. You must acquire enough of these items to have -20% Speed.

The Class "Old" starts with a base -10% so using him can give you an easier time getting this achievement

At the end of waves in the shop look for any item that says -X% Speed, looking at your stats on the right hand side you can calculate how much more you need for this cheevo to pop.

It’s easier if you just use the Old man and pair him with 6 rocks that will give you the extra -10% speed using this method I got the achievement by wave 4