NOTE: THIS ACHIEVEMENT CAN BE MISSEDThe Achievement is for you to survive in the arena for all 8 waves, and not go into the cells once. [The Cells are where you can take a break between rounds]
My recommendation is for you to stock up on Heath, and Might/Will potions, and resurrection potions.
Another recommendation, purchase the Divine Furry (If you are light, if you are dark, purchase the opposite spell. Sorry, I forget the name.)
There will be 8 rounds, Wasps, Hobbes, Balverines, Undead, Bandits, Earth Trolls, Rocks Trolls, and Arachanox.
[Note, that there are traps, spikes, and spinning blades.]
For the wasps, they're simple, use magic, or melee (recommendation)
For Hobbes, I recommend using traps as cover for you bow, going full out, and blocking with your melee, and of course magic. (Side note: Slow Time spell works great with Hobbes)
Balverines, during this, Whisper will arrive to help. At first, you'll fight just a wave of normal Balverines, I recommend magic, and going with either melee, or the bow. Once you fight off that maze, a white Balverine will arrive. For this, use the traps to your advantage, for you will also have other Balverines being a problem. Lure Balverines into the spinning blade traps, and hit them with your bow/magic. Another good tactic, is once the other Balverines are gone, go to town with your melee on the white one, for you'll be bale to hit him over and over. (According to my experiences) After that round, more Balverines will show up, and two white Balverines. I recommend same strategy.
After this, you'll get the Undead
for the Undead, I recommend Melee, and magic.
Once completed with them, you'll now face Bandits. Use the traps, and your bow/magic.
Now it gets easy for the rest of the show.
for the trolls, both Earth and Stone, hide behind the spinners, and dodge their rocks, while hitting them with your bow, after some time you'll get them both ;)
as for the final boss, simply dodge out of its way, then hit it with your bow/magic. When it charges its tail, you can, A. Hit it with your sword when it stomps into the ground, or B. Shoot it with your arrows or C. Use magic.
Then you're done! Hope this helped!
Good hunting!