The Legit Solution - Join a game (Public or Ranked) with four or more players in it (yourself will make player five). Choose a plane that you are able to fly expertly (try for something with high speed, agility, or armor, or any combination of the three). Play the entire game gaining NO kills and NO deaths.
The Boosting Method - Gather four other players (or more). Setup a Public match and invite everyone in. Any map, lowest time limit, five kill limit. Your teams can be setup as such (for five players):
Team 1.....Team 2
....A..................D
....B..................E
....C....................
Player A will kill both players D and E. Players B and C will unlock Spectator as soon as the match ends. Now change your teams:
Team 1.....Team 2
....A............B or C
.B or C...................
....D.......................
....E.......................
Whichever player is on Team 1 (Players B or C) will kill the player on Team 2 five times. This will unlock Spectator for players A, D, and E.