Shalltear didn't need to hit him for very long or do very much damage and she recovered significantly. The hit Ains received was incidental at best because of the mini-teleport. Her recovery capabilities with that lance are no joke.

I assume it is based on percentage of total HP, because she had to kill quite a lot of her summoned critters to fully recover the second time.

Taking anyone else with him really would have made it much harder to beat her. Especially the strong vassals.