I'm playing on hard mode and I'm up to the boss fight in Lhant against Richard. He heals for like 7k whenever he's under 10k periodically, and it's totally killing me. I have no idea how I can do that much damage in that short an amount of time. What's the secret.
Welcome to boss fights on higher difficulties. It only gets worse from here.
Basically what you might have to do is spam artes that will hit his weaknesses. Also, he's probably healing by using the arte that attacks a party member and gives him HP (forgot what the name of it was). Outside of that and using an All-Divide, that's pretty much all you can do.