Has to be WDM Dowling. They're one of four undefeated teams left in 4A and their adjusted opponent's win percentage is .813. For reference, CR Kennedy's is .650, Bettendorf's is .593 and Fort Dodge's is .250. The only team who's played a tougher is schedule is Urbandale(.842).
Adjusted Opponent's Win Percentage: (Opp Wins - Your Losses) / (Opp Wins + Losses - Your Wins + Losses)