The first story boss is one tough cookie, but there's so much you can do before facing him.
Beastman is the first boss outside of the tutorial you should face. You'll find this reasonably easy foe in Groseside Cave, a little to the north of the Church of Elleh.
This strongman hangs out in Murkwater Catacombs, and luckily, he's not nearly as intimidating as he looks. Get his hammer attacks scouted, and you'll put him away with ease.
A skeleton in a rickety boat that's about as agile as the Titanic trying to parallel park? What's not to like? Go to Summonwater Village and fight this miniboss on Torrent.
Found in Murkwater Cave, this guy isn't as much of a boss as he is a fun NPC and entertaining nuisance. Spare him, and he'll become a handy merchant later in the game.
Black Knife Assassin is yet another, 'just some dude with a weapon' boss. Head to the Deathtouched Catacombs and you'll find this slinky but slight warrior.
This sentient chess piece lurks in Stormfoot Catacombs, and its movement is freakier than most other bosses. Still, if you block well and be patient, it will fall easily enough.
Fell all the other bosses on the list and you should be ready to take on Leonine Misbegotten in Castle Morne. He's quite aggressive, but entirely beatable if you show patience.