For Rathalos Ex, your best bet is to just go ahead and get to level 80 /finish main story, and then put a Party Finder up for it. Get a small group going. It's actually soloable unsynced by jobs with low GCDs, but quite easily duoed at 80 and you still get scales and all of the drops, so it's in your best interest to go ahead and get up to 80 to make the farm fast. You'll need a lot of scales and Ex is the best way to get them, and the only way to get the dyeable versions of the armors.