If you still haven't gotten it... I used to have trouble with the Rathalos/Rathian... I went to the training arena and kept gettin wooped by it and eventually got better... I'd advise doing that if you can... also a few tips on killin it with longsword...
firstly, never run at it head on... I'd advise against that for almost every monster, but that can leave you open to it's charge/fire attacks/spinning attack...
start by running round the sides of it... go in at a good angle, you wanna be able to draw hit, then roll at a 45 degree angle to be right out the way of it's spinning... never try to stay in there for more hits... if you do... it may work, but more often than not you'll be on a one way trip to base camp via the kitty cart.
I'd advise going for it's tail, gettin rid of that means less getting hit by it's tail, if you can roll out the way. once it's tails gone aim for it's legs, keep hitting and rolling out, I'd say no more than 2 straight hits before moving... mainly 1... if it flinches take another... if it falls charge up on it's head or tail if it's not off yet.
this is the best bit... when it flys up in the air... stand right under it's shadow and once it's done it's attacking start a charge attack... as it comes down ***SMACK!!*** then once again roll out asap, if you're too slow it can blast you with fire while diving back.
some say it's cheap to use traps and flashbangs... I agree, it's more fun to take it out without them... but when you're having trouble with something it's usefull to use traps... shock traps are great against rathalos... get it to run onto them (watch out for it blasting fire when you're stood infront of it!) once it's in there it's tail is in the perfect place for you to charge attack it off... you can get a few more attacks in after as well... when it breaks out you wanna off out to one side of it... might scream/blast fire... god knows what it'll do... but if someone slapped you with a sword in a trap, you'd be pretty pissed too, lol.
As for flashbangs... my personal advise is, only use it to stall the monster and keep it in one place... use it to sharpen your sword, or if you've taken a lot of damage and need to heal up fast... using them over and over makes it very boring and also makes them very unpredictable/dangerous.
one last thing... if you're in the right place when it blasts the fireball straight on, run to it's head from the side... you can draw attack and roll straight on out the way.
Main thing is not to be greedy/careless... never carve the tail untill it moves area... only take meds/use items if you know you have enough time... if not then just dodge it's attacks untill you have got time. learn it's attacks and you can learn to avoid them... block it's scream if you have your weapon out... if not dive out the way.
... hope this helps... also I find it more fun to play online... PS3 + Adhoc party = pure win.