(a) The figure of speech used in the line "all is fair in love, war and debating" is a metaphor.
(b) The metaphor emphasizes the idea that debating can be intense and competitive, like love and war, and that anything goes in order to win. It creates a vivid image in the reader's mind of the high stakes and cutthroat nature of debating, and suggests that debaters will use any means necessary to come out on top. By comparing debating to love and war, the metaphor also suggests that the outcome of a debate can be just as important and life-changing as the outcome of a romantic relationship or a battle.