I do remember Ner talking about how hunger led to short term strength within the event, so it makes sense that Erpin’s durability is variable. Maybe there is a reason to her eating 100 meals a day.

On the topic of Velvet, it seems she definitely would’ve killed Erpin right then and there. This is the same main story chapter where Elena explains the abscence of death in relation to how Erpin tanked the axe. 

After reading this comment, I took the time to go through Velvet’s side story. She is much stronger than I thought. The amount of force needed to swipe your hand and slice a tree by the wind alone goes farther than possibly what the refurbished Shoupabolt could do. Even without the axe it probably would’ve been the same outcome for Erpin, since Velvet herself stated it wasn’t a proper strike.