Make a green/white deck. This is mine:
2 Mycoloth
2 Mentor of the Meek
4 Raise the Alarm
4 Triplicate Spirit
3 Banisher Priest
2 Jade Mage
2 Selesnya Evangel
2 Doubling Season
2 Planar Cleansing
3 Wall of Omens
4 Elvish Visionary
4 Cultivate
2 Captain of the Guard
10 Plain
10 Forest
4 Selesnya Guildgate
Just build up your forces until you get 100 creatures on the board. Then cast Planar Cleansing. Killing your own creatures counts for this challenge. It can be done in any mode, even against a dummy deck in practice mode.
P.S. If I forgot a card name it is because I'm writing this before I had my morning coffee. Please remind me in the comments below. >°<