If you're looping over something to collect items into an array, I'd rather you use map.

@turns = 13.times.map do |x|
  turn = Turn.new(@opponent_a.strike, @opponent_b.strike)
  if @opponent_a.strike == turn.winner
    @opponent_a
  else
    @opponent_b
  end
end

@turns.each_with_index do |winner, i|
  puts "#{winner.name} won round #{i+1}!"
end

I also separated the running of the turns with the reporting of the turns.

Got it. So in this case I'm interested in getting an array of winners for a given round. map returns an array so it more directly conveys what I intend to receive. Whereas in my implementation, I went in a round about way to get there by separately instantiating an array then using an iterator to populate that array. Is this a good way to look at it?

Yep exactly. 😃