I have been working on Avatar 4e for a little while now alongside Legend of the Five Rings 4e.
In L5R, we built the characters with the standard 4e rules and changed the power names and flavor text. Try to tell me that a dwarf fighter is not a perfect Hida Bushi.
For Avatar, however, I wanted...
You can use Cleave as long as you "drop" an opponent. It does not matter that you used a Whirlwind Attack; you dropped the last giant so Cleave entitles you to one free attack against one of the other 7 giants.
If you had Great Cleave, you could continue making attacks as long as you dropped...