I'm on the urgent slay the rathalos mission and I really need some help..

Right-e-o. Your armour is fine. Your weapon is alright, though I recommend something water.

Take with you these items:

Flash bombs (flash bug and bomb material)
Pitfall trap (net and trap tool (net: spiderwed and ivy))
Potions
Antidotes
Power juice (rare steak and catalyst)
Lrg barrel bombs
Paintball
Psychoserum

The flash bombs and the pitfall traps will help you to get in extra free hits.
The potions are obvious, so are the antidotes, so if you get poisoned.
The power juice will stop your stamina from decreasing, so you can run and block easily.
Blow the lrg barrel bombs on the rathalos when it is stuck in the pitfall trap.

The main times to attack a rathalos is after it has fallen over after a charge and when it throws a fireball at you.

When the rathalos flies up into the air, stand underneath it's shadow. This means it won't poison you or hit you with fireballs. You just have to know when to move, before he lands on you. Either look out for the fireballs (when one or three have been thrown) or listen out for a funny noise. It will make a funny noise when it attempts to poison you. After that noise, then move out of the way.

If you see it limping then run straight to area 5, it will try to sleep and regain it's health.

Make sure to bring along paintballs and Psychoserum, because if you loose it, it could mean failure because you are out of time.

Taking off it's tail also helps, it makes things easier.

An added tip to ieatwyvern other than taking the things he said instead of a greatsword take an SnS(sheild&sword) at your level you should probably get a poison battle axe don't mistake it for the poison axe other than that you armour is perfect I believe it has a sharpness skill which will be great since I beat it with posion battleaxe with just the items it will be much easier for you