Skye's Ultimate can be devastating to your team's collective health. But you can be as close to it as you want as long as you break line of sight. If Skye doesn't put the Ult on the Payload, you can you said payload to block ALL Damage from it.
You can encourage him have to get close enough to shoot by using the shield Legendary to block shots and give him room to advance. If he tries to contest you on point then you can use Kinetic Blast to knock him off and use your shield to stay safe.
If the enemy Ash doesn't pick up Resilience or the "Battering Ram" Legendary, then you can punish her for it by saving your hyper beam to knock as out of her ult's safety circle. This will often waste her ult or her movement ability.