Every team must try to play with their best but Champions League is a very tough tournament and in this tournament if good quality players cannot be formed in the team then competition with other teams can't benefit. Arsenal have quality players but a clear weakness in their finishing positions.
The champions league has become very tough tournaments and in this case every teams have to been prepared well against next season, if any of the really want to be the part of those that will participate champions league next season. Arsenal have a strong Star players but is there inconsistent that is demoted them, and that's why even when their performance is improving and proud that they want to move forward into better squad they later relax that only arsenal problem.
Their defense should be a little stronger in addition to the finishing position being a little stronger. If they can do well in these two positions, they will have a good chance of doing well in the Champions League next season.
I think is only few of the arsenal positions that are link if they the others position are doing very well, because even if we look at there defense arsenal defense are try there possible best to maintain their defense position, expecially Saliba the guy is strong player which evey of the Arsenal fans are proud of him. The only thing that Mikel Arteta can do to make the team very standard them before is for him fetch out those positions that are link and put the strong players in that position.