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. 
I won't be suprise if Arsenal lose to Brentford, I think Arsenal should really be prepared, most people believed that Arsenal will win their last match which they lose, anything can also happen to them tomorrow, we shouldn't underestimate any club currently. If not that Manchester City also lose to Tottenham in their last match, the point which Arsenal will have been using to lead Manchester city will have reduced. I think Arsenal should do everything possible to win their match tomorrow, if Arsenal can lose their match tomorrow, then I have confidence that the league those not belong to Arsenal this season.