Complex sentence: The team that Manchester United has met most in league competition is Arsenal, against whom they have contested 186 league matches, having won 77 of these matches, which, alongside the league wins over Aston Villa, is a club record.
Split into simpler sentences: The team that Manchester United has met most in league competition is Arsenal, against whom they have contested 186 league matches. They have won 77 of the matches against Arsenal, which, alongside the league wins over Aston Villa, is a club record.