Depends, but the toughest for me was Bonemass, mainly because I was under geared. It took me a long time before I even found Yag and by then I had plenty of silver and black metal. Moder was also tough, but also fought with good gear. Bonemass was fought with troll armour. I don't think it was upgraded due to the requirement of obsidian to upgrade the work bench. Laziness/tedium in mining kept me from getting enough for other armour pieces. ...and the swamp area in general is tough to fight in.