Look, such games are expected, that's why you take care of the other games, as Oly has done so far. It'll come down to the game at Karaiskaki, which actually it's a great thing. Let the best team go forward.
But, don't expect Arsenal to be the same team you beat in the first leg.
Unfortunately, the gap between the very rich teams (like Arsenal) and the rest of is getting wider. A few million euros difference in the budget probably isn't the determining factor, but the gap is in the many tens of millions. The half a dozen big teams can spend for 1 player as another team spends for an entire roster. Even in Greece it's evident, when Indeye costs more than the budget of entire teams.
As for your game against Arsenal, I don't know what the best strategy would be. Try to not lose or try to score at least once? Tonight, I thought your defense didn't do well, not because of the 4 but too many mistakes. If this is repeated, Arsenal will plenty of chances to score. I think you should demonstrate power; they must respect you (and fear you) since you beat them at their home. If you appear weak, they'll step all over you.