Plesioth is a wyvern that most veteran hunters will know by heart. It has been around since the first Monster Hunter game and it re-appears in MH3U. It is infamous for its Hip-Check attack and its love for frogs.
Hunting a Plesioth
Being that Plesioth first appears as a High Rank monster; you most likely have reached HR3 in the Guild. If you haven't already, fight Rathalos until you get a full set of the 'los gunner armor. (If you don't know how to fight one, go here) Gemming this set will give you unbelievably good stats and skills to fight Plessy. Attack Up (M), Pierce Up , and Fire Attack (1); all three will bring out the full potential of the Arco Unu (R). While the official weakness is Lightning, Fire will work almost equally well. Since this is a High rank mission and you will be in Low rank armor, used the Armor Spheres to bring your defense up. This is critical as a hunter in non-upgraded 'los armor will get destroyed by some of the attacks Plessy can dish out.
Item wise; bring the standard hunting gear with an option to make super potions when you run out. Para traps work underwater so if you plan on capturing, use them.
If you are doing the HR3 quest, then you will be in the Deserted Island (N) area. Go to area 10, he will be there. You should also get used to fighting in that area as 90% of the fight takes place there. If Plessy doesn't spot you and he's in the water, try to fish him out with a Frog. You will do some damage and get him on land, flopping on the deck. While fighting on land, use the Arc Shot to attack his top fin, this is his weak point and can be broken for extra rewards. Also take aim and fire a Piercing shot down the length of his body for maximum hits and damage. The major attack you need to keep an eye on is his dive charge. He will drop on the ground and "swim" towards you at a high speed. His wings will most likely get you so be sure to roll far to the side to avoid the attack.
A MAJOR note to all veteran hunters who think they know what’s what about the Plessy; the top fin is no longer the damage indicator but it is now the stamina marker. Plessy now has a "limp". /note
In the water, Plessy is all new, (mainly because you couldn't fight it in the water before). Its attack pool is basically the same as on land, with the added jump splash attack. He is in many ways easier than on land as he can no longer do a charge attack as effectively. Mainly use the Piercing shot down the body like before and you should get him hurting bad in no time. He likes to jump out of the water if things get too hot, so be prepared for more land based fighting the underwater.
When damaged, he will retreat to area 11, then 12 to rest. Trapping him underwater is tricky as he will be less inclined to charge at you.
So you finally hunted the Plessy, and what did you get in return... well, not much in the ways of an archer. The armor lacks the luster or stats that other beasts as difficult as a Plessy might give, and the Bow is a bit far off to worry about just yet. So what did you gain by fighting the Plessy?
Well, err... um... if you are looking to broaden your horizons and look for a new weapon class to play with at a relatively low investment, the HeavyBowgun that Plessy gives, the Pressurized Phaser (great name BTW), is a great weapon to do that with. It can use most lvl 1&2 ammos and its damage is pretty good given the time you are able to earn it.