Solution from Xbox version.
I got a Witch Doctor and Barbarian to 70 and was never able to do this. With the Barbarian and whirlwind I came close but I'd run out of fury and be unable to get above 30.
How I finally got it was with a Wizard, and I did it very early on. I was level 10 and doing the skeleton king quest. At the beginning of the Royal Crypts I used Wave of Force (Y button skill), It uses hardly any mana and has a huge radius. I just ran around the area from the bridge where you enter, pressing Y every second. The trophy popped before I even realized I had gotten more than 50.