Since Byleth possesses Divine Pulse, which can manipulate time, she might be able to resist Dimaria's Age Seal, not certain, while the same can't be said about Dimaria being able to stop Byleth's Divine Pulse. Age Scratch will still be very much be absolutely devastating if she lands it on Byleth.
There is Sothis' Shield, which grants Byleth an increase resistance to magic for a limited time, allowing them to resist Dimaria's Magic. However, they can only use it once
If Byleth gets low on health, she have Defiant Strength, which increase her strength, and Vantage, which increase her speed
Byleth also got some of ways to heal herself, like with Vulneraries, their Crest, and Healing Focus
If Byleth has accessed to Optional Equipment, she can handle the range gap with Chalice of Beginning, get Damage Reduction via the defensive Heroes' Relics, etc...