A lot of poeple have been having trouble with this challenge so I decided I would share the strategy that worked for me.
Wave 1: Consists of a single Cyclops. Using the   /\ combo with Thera's Bane plants a bomb that will go off in several seconds and this is very effective on the cyclops and gorgons.
Wave 2: Consists of 2 Gorgons. As soon as you kill the first one wave 3 will start.
Wave 3: Adds another gorgon to get the count back up to 2 and also has 6 Fire harpies. In my opinion this is the wave that will give you the most trouble, and i recommend with the combat arena to familiarize your self with these guys. I think that using the grab attack is the easiest way to kill them but becomes risky if you do it more then once in a row. If you leave any gorgons they will continue onto the next wave.
Wave 4: Consists of 6 Cursed wretches (the tiny spiting spiders). These nasty little things will spit at you and if you hit by a single one game over. the good thing is that they are blockable and have a instant kill grab. Even though you can parry and through the spit back for a kill, most of the time it will just leave you open and will result in you getting hit.
Now for the Actual strategy:
Deal with the cyclops however you want but i would recommend picking with off with   /\ and keeping your distance. After he dies the gorgons spawn and they atually very rarely attack you. they will stick to their grab attack (which if you do minigame right you wont be damaged) and their stone attack. Damage the first one until he if able to be grabbed for his finisher, and then switch to the other gorgon and kill him. This will spawn the first 3 of the fire harpies. Run over to the remaining gorgon and grab him. When you rip off his head you will stone everything around you and the first harpies will fall to the ground and shatter. Run over and quickly break the other gorgon that spawned and that will be the end of them. 3 fire harpies will be left, After you take care of them the spiders will spawn. Immediatly start blocking, when your blocking the spit wont damage you and they will move towars you to jump on you. Grab them when they get close, If you can chain grab them do it! They wont attack or spit at you while grabbing. After youve grabbed all of them you can block and repeat until all are dead.
Hopefully with this you will be able to get past this challenge without ripping too much hair out! And if you need any help on any of the other challenges post and ill try to help you as best I can. Good luck