Never used the dual swords against a Rathian, but that might be your problem. The demon mode dance leaves you extremely vulnerable if your using that, especially if your trying to hit her head with it if she's not tripped or KO'd (punish Draw). She moves much faster than green plessy and that kazumaki hurts big time (ironically I hard harder time with G.Plessy taking me nearly 43 min to kill the thing my 1st try -_-)
I think I slayed her with a CQ's switch axe (as well as using high rank Narga armor) on my first try, but I'd also recommend the hammer where you can charge it up then strike when she does dumb stuff (shoots fireball), and obviously aim for her head :p
You sound like a person that watches Octaneblue's MH3U videos.
To add onto what he's saying, to avoid the poison better, look for a pause in her actions, usually she will fly for abit, make a 90 degree turn and then tail flip, if she misses, she will usually do another tail flip after another 90 degree turn with almost no wait, so start rolling to the side when you hear that wind blowing (raths flying around sounds like wind) Also, when she charges and pauses without facing anything, she will usually do the spiral tail flip into the air and face her target usually tail flipping again. I just get my attacks in when she's done her triple charge.
"Why do I check for updates when I know there's nothing there?"
Bowguns ROFL stomp the Rathians IMO i had trouble with the first one i fought in regular rank just to get used to them. now i can kill them stupidly fast in any rank rather it be azure Pink rathalos or rathien. if you dont commit to to much you can honestly ruin them with out taking a hit and you dont even need the evasion skills either (which i really need to go get >.>)
ErrorSupply 3-D DS -OMG! It's the ultimate anti-pirate tech O:.
I still cannot beat her. The problem is the poison. She poisons me, and I'm getting up with only a spec of health left, only to be killed immediately by the poison. And after that, I'm back to 100 health without ancient potions or nutrients, which keeps getting the quest failed after I die 3 times of lack of health. I need felyne foodie -_-.
Gem in poison immunity, seriously. Whatever your other skills from gems are, it's worth losing them just to get poison immunity against p.rathian.
And bring max potions and materials to make more if you're going through them a lot.
Without the need to worry about the poison, it's just a rathian.
The first G Rank armor I had made was actually the ludo z armor. With negate poison and hunger I didn't have much trouble handling her annoying swooping tail strikes. The only thing I had to worry about was those fire attacks and that ever so loving fire bite she does nearly after each charge. Weapon wise I stuck with my dios slicers+ and with patience. and a lot of distractions from my two lil buddies we took her down easily. Hate that fire bite attack -_-
I killed it with a Dios+ SA and Volvidon S armor, 1st try... and it was my 1st G-rank fight. Take antidotes... you'll need those, but you already know that :) You can always tell when she's doing her spiral tail flip, so just roll away. And the only other rule is : NEVER STAY IN FRONT OF HER ! (applies for all rath fights) Always be at something like 135° from her face (that means 45° from her rear). And watch out for tail swipe.