I take it, that you're playing as a Cleric. There are few ways you can do this:
- if you have intervention skill, then use mana potions. 35 mana is 280 health, so that should be enough to eventually outtank him,
- position yourself in a way, that will allow you, or your companion to have flanking damage bonus,
- if you have 3rd level of Arbenos might, then consider using maul of terror,
- as mentioned above, scrolls of circle of restoration are pretty useful if your companion falls in battle, and you need him/her to help you right now, you can buy those in loreseekers hall in new garand,
- Underking has strong duel skill, you might consider using some tricks to switch his aggro towards someone else when he's getting dangerous, or stunning him(choose between Adaons sneak attack skill, or Grisenda two handed expert, Bash, or whirlwind skills),
- Don't get stingy with elixirs of Galade, you can always find more,
Have I missed anything?