If Arsenal is going to beat Bayern Munich and take revenge for all the beatings they have received in the hands of the German champions, then the time is now, they might never get a better chance to beat Bayern Munich. Tuchel's men are really struggling and now their only chance of lifting a trophy is the CL, but they are faced with a huge test in Arsenal.
If Arsenal are to progress to the next round, then they have to beat Bayern at the Emirates. Bayern Munich might not be in form, but beating them in the Allianz Arena is a tough proposition, thus Arsenal need to take the initiative and win in the first leg, so they can at least carry a win into the second leg in Germany.
It would be more advantageous for Arsenal if they played their first game in Munich at the Allianz Arena, but they should still qualify for the next round. FC Bayern Munich is currently very weak and Arsenal is very strong, if they don't make it now they will probably never make it again. But they should win at home and a draw away would be enough.