As Orisa I always try to keep my barrier between me and the enemy, since I have a huge hitbox and I am very fragile without the barrier protecting me. Since my barrier is a stationary shield that blocks damage traveling from any direction, in the situation you described I would walk forward in front of my barrier, placing it between me and the enemy Genji. I would also probably throw halt on the ground on the other side of the barrier and etch out another 1-2 sec of time while trying my best to get headshots on you. Orisa is horrible in 1v1 situations, so it's realllly important to secure headshot "clinks" against flankers and force them to retreat or even get a kill. I also have Fortify ability which reduces 50% damage but has a long cooldown so I would only exhaust it as a last possible recourse.

All in all, Orisa has horrible mobility, a huge hitbox, and is very fragile without a barrier protecting her. That is why Orisa players need to play with their team to avoid the situation you described.