First time posting a solution. I found this on x360a
http://www.xbox360achievements.org/forum/showthread.php?t=31...If you missed the opportunity to read the first solution before entering the Crypts in Stonebridge, You can then head on over to Glitterdelve Mines (the Kaari Pass) you can simply stand on a fissure and it will do over 400dmg every 6secs.
This method is only slightly worst than the first one mentioned.
I am playing as Katarina and as a range char it is very hard for me to take 100000 dmg in a session because most of the time im just running away from enemies and sniping.
Hope this helps and if I can make the solution easier to understand do let me know.