Can you interfere and help and win the game as an elf in munchkin?
According to the Munchkin FAQ under Important Note #2: Playing/Using/Switching Items During Combat:


  Playing/using a one-shot Item during a combat that aids the munchkins (and not the monsters) does not constitute "helping" in combat, as described in the rules. This means that in any given combat, any number of players can play cards to aid or hinder either side as often as they want, as long as they have the cards. Only one other player can actually join the combat as a helper, however.


So, no, this would not actually constitute "helping" in the combat to satisfy the Elf racial ability.

However, if through some other means you had actually been the helper in the combat, then yes you could have won or tied, as long as the card that allowed you to help did not specify that the level gained could not be the winning level, because according to the FAQ under Important Note #1: Reaching Level 10:


  Note also that ANY level gained as a result of killing a monster counts as the winning level.


And also from this post on the official Munchkin forum


  The level the Elf gains for helping in combat can be the winning level, because it's a level gained as the result of winning a fight.