A tilting horizontal cutting blade works quite well see Variant below.
http://i109.photobucket.com/albums/n66/PeterW_2006/DSCF0467.jpghttp://i109.photobucket.com/albums/n...6/DSCF0467.jpg
It came second in the last world series event and can be seen flying out of the arena below but not before the other robot exited in two directions.
http://i109.photobucket.com/albums/n66/PeterW_2006/flying.pnghttp://i109.photobucket.com/albums/n...006/flying.png
Try the forum on http://www.robotwars101.orgwww.robotwars101.org for all things antweight