Here you can find some useful hints and tactics to beat second boss.
Guide to Beat Second Dungeon Boss
Second phase – you need to realize that when it does the charge attack, it tends to do it twice. If you bait out the second charge and dodge, you’ll be able to get a free hit or two on it while it recovers. This should be one of the only times you try to melee it. Make use of your dash to skirt around the arena – even though it is faster than you, you should have enough time to dash away before it actually tries to attack you. The axe is fine for this purpose, you won’t have long to hit it so the axe is strong enough to make all hits count.
Stay as far away as possible when it rears up to shoot lightning everywhere – this will give you a bigger safe zone to dodge. Using fireballs is a good idea.
When you take down the second form, immediately start eating/healing as necessary to be ready for the third form, which is more of the same.
Tip: Random chests sometimes appear instead of monsters, opening these may occasionally net you absurdly overpowered equipment for your current point in the game.
General advice: When in doubt take 2 villagers with you or 2 of your monsters. Villagers have the advantage of occasionally healing you.
Note: During dialogue there’s a button prompt (L/R) on controller. Press that and after the normal dialogue you get the option to invite them to join. They only join if they like you enough and it fits their schedule. Forte can join from the get go though.
P.S. You can also just grind in the 2nd dungeon for a few days. Eventually as you level up things stop being able to damage you, even on Hell difficulty.