I think somewhere over 80.000 will allow you to kill them easy. It may depend on your weapons a bit. Anything Hydro will face repguana if you have, as I mentioned, 80k+ damage span. I'm not very sure because this is how I just remember when I was a mage.
Anyways, there's a website which help you, and not only at the damage part.
http://koalasium.blogspot.com/p/damage- ... s.html?m=1