Royal Rat Authority

At the start of the battle, prioritize taking out the smaller toxic rats. They die easily but one hit from them will inflict toxic status on you, which is worst than poison. Once you have this status, it will almost impossible to recover. If possible, try to use ranged attacks to kill at least 2-3 rats, depending on your attack speed. Switch your melee weapon afterward and take out the fourth.

Once all the small rats are gone, you'll have to face the giant rat itself. It only has two main attacks; one is the unstoppable, almost unavoidable lunge and a very wide swipe using its two arms. The lunge is pretty tricky to avoid. It seems the most effective way of preventing damage is by blocking it with a 100% damage reduction shield – at the expense of your stamina.

Your best chance however, is when the boss starts its swipe attack animation. Roll towards him so you'll be under. If you've done this during the first swipe, you'll have more than enough time to heal yourself or land an attack or two. It seems that the boss will be forced to use its swipe attacks if you keep yourself near him. Since this is the only attack you can exploit, have your shield up and keep yourself close to him to force him to use the swipes.

When the boss is near death, it will pause briefly and begin to vomit toxic sludge. Stay away from its mouth and near upper body since that's the area the sludge covers. Make sure to use this chance to deliver the finishing blow and secure victory.

Hunter hunted