This is a main story mission and will be reached during natural progression. You will reach them at the end of Infierno on the way to the Templo de la Serpiente. You'll be told by the Chicken Pope that the "bird who must not be named" needs to be defeated and then you'll go through the large cactus-shaped door. The fight is relatively straight forward and not very difficult.
First, attack Zope in any preferred method until he disappears and sits on Cactuardo's head. One of the walls will have needles shoot out of it and you can tell which one will as it will shake just before the needles appear. If you're on the side of Cactuardo that doesn't have the needle wall, simply Wall Run up and Wall Fly over before Frog Slamming onto Zope. If you're on the wrong side, wait until Cactuardo jumps then run underneath him to the non-needle side. Be careful as he shoots spikes along the floor once he lands his jump.
Once Zope is knocked off, Cactuardo will be stunned and you can attack him however you like. Eventually Zope will return and you cannot attack Cactuardo again (his needles now no longer retracted) until you Frog Slam Zope off of his head. Depending on how much damage you deal out when Cactuardo is stunned, you will likely only need 2-3 instances of knocking off Zope before Cactuardo is finished and the achievement will pop shortly afterwards.