Yes, an elf can win by helping
http://www.worldofmunchkin.com/faq/
Important Note #1: Reaching Level 10
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Note also that ANY level gained as a result of killing a monster counts as the winning level. (If a card says a monster is automatically defeated, this counts as a kill unless the card says otherwise.)
If you have a Race or Class ability that lets you get an extra level after winning certain combats, that level can be the winning level. A Go Up a Level card played after combat, however, cannot be, because the card does not itself kill the monster.