I feel your pain OP.
I'm currently in the middle of leveling my rogue (89) and warrior (88) up to 90. My warrior is literally 3 shotting mobs and taking on + 4 at a time, an absolute breeze while rogue struggles when I unfortunately pull more than one mob, it so painful it's unreal. Far more than it used it be from what i can remember.
My advice to you would be to go combat, get correct glyphs, especially DM, and if you happen to have a 90 already farm the hidden treasures you can find around pandaria. I know for a fact there is a bind on account blue dagger, which you can use at 86, with a very respectable ilvl available (beleive it is found in the north east of dread wastes, check wowhead) that you will keep until 90.
Other than that, plug yourself in, get a good music mix and grit your teeth to level 90. GL