What I usually do is before starting the Lightning Round, drink a bottle of Guardian Potion + AND have the potion medal (so that it lasts longer) in your pouch before you speak to Thunder Dragon, I can take no damage for the first two or three battles before the effect stops working.
^ Interesting. Maybe I'll save that idea for the next play through. Question though, how would the potion medal be of effect if you can't access your adventure pouch? Meaning, you also don't have your extra life medals when you are in the battles.