Niffelheim

The bosses in this game look like the standard/normal enemies (Bears, Boars, Spiders etc.) but they are much, much larger and stronger. They are found in the large open rooms in the underground areas. For these 3, you must kill Bosses in cave/dungeon areas. The lower you go in the caves/dungeons, the more difficult bosses you'll find so you need to make sure you are well prepared to defeat them.

It's recommended you not even attempt to take down one of these with anything less than steel-grade/titanium-grade Weapons and Armor. Make sure you also bring both healing and strength potions along as well as any Charms to increase your defense and/or healing. The HP of the boss ranges based on whether or not they're wearing any armor (you'll clearly be able to tell if they are or not, it's not everyday you see a bear wearing an iron helmet).

Each type of boss (Troll, Animal, Spider and Undead) has a Brew you can take to increase your damage against them and decrease the damage you take from them. These are incredibly beneficial so it's highly recommended you keep a few of each type on you at all times so you can drink one whenever needed.

Once you kill one, it will respawn after a couple of days in-game time. You can kill it again and it'll count towards this trophy but just a heads up that the Boss(es) will get stronger each time it comes back. You'll need to kill 3 anyways as you start collecting the shards required to open up the Portal to Asgard (you'll need to kill at least 20 Bosses for those).

Once you have killed your 3rd underground Boss, the trophy will unlock.