You're right, AS Roma just have to hold Atletico Bilbao to a full time draw win the match but it's risky since they'll be visiting Bilbao's home for the second leg, that's a hedge to Bilbao since it's their ground and they'll play better to impress their fans.
Getting a draw is enough for AS Roma and they have 11 unbeaten streak in Serie A, with five latest matches are wins. It's very impressive form that will be brought to Bilbao for the second leg, but it won't be an easy match for AS Roma.
Athletic Bilbao with their match in Rome last week, shown that they are strong and if they did not have a red card, they would have left Rome with one point. Anyway, a defeat with a single goal in Rome, is not too bad for Bilbao and by playing at their home stadium in a second leg, they can find a win and at least equalize an aggregated result within 90 minutes.
This match can be very exciting and there will potentially two extra half and maybe penalty shootouts too. These teams are too balancing in quality and not too different in their forms recently.