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. 
Arsenal losing their previous game against one of the small club in the premier league, and facing Brentford wouldn't be a big deal for arsenal. Brentford are in a good position to face the gunners but it's definitely going to be one tough big game for the bees who will be visitors in the emirate stadium. It's the right time for Mikel Arteta to face Thomas Frank in the league game, with arsenal on top of the table and Brentford ranking 7th spot on the table. I'm placing my bet on arsenal because I'm very confident that they will win Brentford with ease.