User blog comment:B214/Shin DxD 1 discussion/@comment-175.138.188.184-20180926082732/@comment-33939194-20180927181854

Each time Crom destroyed a part of Vali's armor, he gave damage to him. ["Don! —Crom’s punch hit Vali who had descended from above, shattering the armour at his abdomen!

“Guha!”

Underneath his helmet, Vali spat out a mouthful of blood."]

As for being a defensive battle, Vali's constantly had to retreat with Crom chasing him down. His speed was useless as Crom could read him with intuition. Half-Dimension only slowed him down but even that didn't work and Vali was outright surprised by it. You say armor could be repaired but if that was the case then why didn't Vali repair the damage Crom did to it?

Being fatigue is a big deal because evil dragons don't care about damage as long as they can still fight. They are very tenacious afterall. This is further shown ["As the explosion cleared, he saw Crom’s body covered and dripping in blood! —But, his fighting spirit hadn’t decreased at all. In fact, he had become even more motivated. Upon seeing that his aura hadn’t faded at all, all Vali could do was laugh"].

As we see, despite the damage Crom has taken, his power didn't drop at all and it only fueled his fighting spirit even more.

Xenovia will defend herself even if she's immortal. Strada could blast her away. Furthermore, any of her attacks can be countered or overpowered by Strada which would drain her stamina. Fighting stronger opponents would drain you morenthan fighting someone on a equal level.