I did this on the shire level in the evil campaign. The trick I came up with was to shoot the ents with normal arrows, so it charges up your fire arrows faster.
The hobbits there are also infinite.
This was also glitchy for me, after about 3 different attempts it finally unlocked, just keep trying.
Victory is but a prelude to the next battle - Naruto + One Piece + Bleach = Awesome - Final Fantasy Fan - Formula 1 Fan - Gears of War Fan