Plsioth might not do much but what he does do is vary painfull.
If you want to do it the safe yet long way you can gun him useing the Blood Shotgun and level 3 pellet ammo
Or if you want to get it done fast you can equip the strongest Sword and Shield weapon you have and go after him. The best thing you can do is attack his legs while he is spinning around to try and tail whip you (Since he is so big only the tip of his tail can hit you), and attack his neck when ever he trys to spray water at you. When you see him lean back, about to try and body slam you, thats when you should roll to the side under his neck so his body does not hit its mark.
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My dragon has issues
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WARNING: "Next left" does not mean "turn left immediately."