How to Defeat Second Boss
I went for the caster first too. I found having a spear helped as you can just do a normal move to kite away while also attacking. I ended up using most of my movement cards killing the caster, and I think I used a grapple to get space then just moved at the start of each turn to kite back after that.
You do need some strong hits before you are backed into a corner though. I had two final blows and two last stands. One is the one that gets stronger the more fatigue cards you have and the other lets you retain the card and it gets stronger each turn(which I saved the entire fight until I was fighting the second guy). I was using them with warcry(50% all attacks for the turn), wind up(double damage for one attack) and power through(upgraded version to get even more strength per fatigue).
I was able to get some pretty big hits in, for like 80 damage in a single shot with that. That still might not be enough alone, but if you can get the caster to shoot the other guy a couple of times, that can help. Throw dirt is a useful card too, since you can outright stop any attack, even if it is just one.