Yeah my first battle with the king went like that. Just got to him a second time and realized they've got 12 thorns, no idea how you're expected to beat the king.
Hover over him to see his abilities. He gains regen each turn, and gains thorns and rage when you hit him. The regen isn't a big deal, but the thorns can wreck multihit strategies like shiv or cleaver. Ironically, thorn+turtle works pretty well against him; with enough defense and at least 2 thorns per turn he'll eventually die if you just don't attack him.