I find hard to believe that Arsenal will lose points with Brentford , but I do see this as a very tough and challenging game for Arsenal just because Brentford are having an exceptional run and season so far that a defeat with Arsenal won't leave much sorrow for the fans. Besides this , Arsenal are mentally prepared to give their best in order to win EPL title this season even if what you said about their latest performance is kinda true because they didn't perform at all at a higher level but it was expected after such a godlike run. 
The probability that Arsenal loses tomorrow's game against Brentford is quite minimal. If they lose yet another game, it will be clear that they are not prepared to hold the title of Premier League Champions for this year. They are aware of what would be at risk for them if they lost against Brentford. They won't be as fortunate this time around as they were last week to see Manchester City lose their match as they did to Everton. They will get to see how Manchester City will overpower them to lift the PL title again if they lose other of their matches by mistake.