Before you start this one, I recommend you bring Red Steve a Red Pickaxe first. See RedStone Pickaxe achievement for more details.
I'm going to assume people read how to go about getting the Red Pickaxe and will now also assume you have changed that same pickaxe to blue using the tree leaves and the colour changer. Once you hand this to Red Steve he will say he can get back to digging and that you are entitled to dig up anything from his treasure trove.. Head on in the now opened door and grab one of the yellow crystals sticking out of the ground. Bring this crystal back to the colour change machine and place it on the platform to be coloured. Go to the drop spot and spawn your purple cube (force cube) inside so the colour becomes purple and use the machine to change the crystal.
Now with crystal in hand run over the carrot field with 3 large purple crystals surrounded by carrot loving flies (just slightly past the carrot field that has no crystals) and walk near one of the crystals, just one will do as you can't collect more than 1 crystal worth of flies at a time. With the flies following your crystal run over to the carrot patch that is full of carrots and just walk around in it.. The flies will have a field day eating all the carrots and the achievement will unlock as you do this.
*Take note of the house near you, (the Reds you just made mad for wrecking their carrot field) because inside that house is a water barrel and our next achievement. "Strong"