Indeed, Monaco is a top team in the French league, of course that's why they entered the UCL. However, to be honest, sorry if compared to Barcelona, their team strength should not be balanced. Barcelona should be superior in all lines. However, unfortunately, it is not because Monaco is very strong, but because Barcelona only played with 10 players from the beginning of the match. It was really difficult, 10 vs 11.
Yes of course it would be different if Barcelona was coached like Pep Guardiola. Barcelona will definitely change their formation more intense to defense and and maximize their wingers. Gonzales slightly pulled in back to help defense while Casado maybe a little bit ahead to help supply a ball to Yamal and Raphael. When I look how their play, that look their defense very open and make attacker Monaco easier to build attack. Because Barcelona have Lewandowski a head, so it's easy to organize players on center to pulled back when Monaco player planning an attack.
It is difficult to create a formation when facing a superior number of players, because it will not be balanced and in my opinion implementing a defensive strategy and utilizing fast counterattacks is the best option. When they build a strategy by relying on ball possession, it will be more difficult because the space will definitely be more closed because they lack players who can create open space to receive the ball.
Honestly, I usually hesitate when a team has to play with 10 people against 11 people, because the strategy used will look lacking because there are no players. Although in some cases the team can win even with a lack of players, but usually the match is very different in terms of team level.