After 12 successive league defeats at the hands of Pep Guardiola's men, the Gunners enjoyed a moment of sweet redemption at the Emirates Stadium.With just four minutes left, Martinelli's shot took a hefty deflection off City defender Nathan Ake, sparking wild celebrations as Arsenal clinched a top-flight success against the treble winners for the first time since December 2015.Arsenal had beaten City on penalties in the Community Shield in August, but that victory in the curtain-raiser to the domestic season could not be taken as seriously as this potentially crucial result.It lifted unbeaten Arsenal two points above third-placed City into second place, leaving them behind leaders Tottenham on goals scored.For the first time since December 2018, City have lost successive league games following their 2-1 defeat at Wolves last weekend.Add in a League Cup loss against Newcastle and City have been beaten in three of their last four games.It is a rare sign of vulnerability from a team who started their bid for a fourth successive title by winning the first six games of the season.Few will expect City to surrender without a fight, but Guardiola has issues to solve as he looks to reinvigorate a squad that may have lost a little hunger after their treble triumph.There is no lack of appetite for silverware among Arsenal as they try to erase the bitter taste of blowing an eight-point lead late in the season to finish five points behind City in second place.Guardiola said this week that Arsenal "are back" as long-term challengers to City's domestic supremacy thanks to the astute guidance of his former assistant Arteta.For once, Guardiola lost the tactical duel with his protege as Arteta's substitutions proved decisive and gave the