They lack this one striker as you guys mentioned and they are playing in the most competitive league in the world. I am sure that Arsenal has a very strong team, but in the Premier League having a very strong team is still no guarantee to win anything there.
I fear that in the Champions League this will become even more evident when they play against teams that have a pure striker. Like Bayern Munich who has Harry Kane or Manchester City has Haaland. Inter Milan has a pure striker. PSG has Mbappe.
Arsenal have always failed to be the best in the Premier League and the last time they were able to be the best was during Arsene Wenger's time. Even though Arsenal has a number of great players in it, because the English league is quite competitive, the quality of the players cannot fully have an impact and this was proven last season where Arsenal lost the opportunity to become champions in the last five matches so Manchester City was again the best.
The Champions League will be much more difficult, apart from having great players, experience and mentality are also quite influential. The opponents they will face in the Champions League will be much better and Arteta may need additional players if he wants to compete there.
Maybe they need some luck to find a pure striker in the next transfer window who is willing to play for Arsenal. They are not the destination that the top top strikers want. Someone like Mbappe would never go there.
There is no such thing as luck in football and I don't see any chance of Arsenal reaching the semi-final stage of the champions league. Mbappe's problems will become clearer when the transfer market closes because so far the rumors circulating about him are completely inaccurate and quite difficult to see the truth.