A helpful advice. Try switching your weapon to a knife, dagger, axe, or things that can be used to "slice" and "chop" when facing a swamp monster. They may have weak ATK damage, BUT they work well and deal great damage for the swamp monster, as piercing and stabbing don't do a lot for them like spears. Don't forget to switch to a different weapon if it's not a swamp monster because, as I said, they don't deal high damage. Also, it's better to switch BEFORE entering the fight than switching while in a fight because enemies might have a chance to hit you after changing.