Ended up getting stuck farming lagiacrus so I could make an Arcus Lance to kill Barioth with. After about ten tries I finally brought him down and now I can consistently kill him - it's a tough fight to learn, but once you know what to do he's honestly pretty easy.
Barioth has a very, very small stamina pool. When it's low, his speed is basically cut in half, and when he's out of stamina he takes long breaks between attack rotations that give you plenty of room to get in there and beat him up. His speed is also dependent on whether or not his wings are still fully intact - he uses them for traction during his attack rotations, so if you can break his wing claws off his speed will further get cut down by another 50%. He seems to ignore his stamina during rage, but not the broken claws. So first priority is to break the claws off, but this is easier said than done.
After your first time seeing, he always starts in area 6. There's a room to the north with Baggis, this is usually the first area he goes after he leaves 6, so wait there and prep a pitfall trap and two large barrel bombs about where you think one of his wings will be once he hits the trap. This isn't an exact science, obviously, but the barrel bombs have a wide explosion radius anyway. Once he trips the trap, blow the bombs up (I used a small barrel for this) and spend whatever time you have left after the bombs go wailing on that wing. Try to aim all attacks at the wing until it breaks. If he enrages before you've broken at least one wing, run off screen and wait about two or three minutes until his rage ends.
He'll usually stay in an area until he's either taken a lot of damage, or been in an area too long. When he's completely out of stamina he'll usually stick around until some internal timer dings off, at which point he'll try to fly to one of the two zones with herbavores in them - they're near the south end of the map. Try to get there first, and if you can, kill or chase away any herbavores before he lands. If he gets one, his stamina is recharged and he'll be moving at full speed again.
If you don't think he's going off screen for stamina - he timed out or you did enough damage, but he's not moving really slow yet - then go to wherever he's headed (hopefully you paintballed, otherwise, eyeball it - he's predictable and tundra is a small zone) and drop a trap on the ground. Alternate between shock traps and pitfall traps, because the duration will shorten each new time you drop a trap. DRop more bombs. If he still has a wing claw left, blow that shit up. Otherwise, drop your bombs on the face to blow up the amber tusks, these are used for all the barioth weapon recipes and at least two pieces of the barioth armor (which is the first armor with Evade on it, and is fucking incredible for anything you fight outside of the Volcano.)
Once all the parts are broken besides the tail, go for it. The tail isn't used in a lot of attacks and is pretty predcitable once you understand it's physics, so I don't consider it high priority for life saving. It's also a fucking durable tail. I've killed barioth five times now and taken the tail off twice. :(
You can also skip the tail and keep wailing on the head for maximum damage. For best results, use a fire or lightning weapon.
Edit; Oh, if you're using sword/shield or lance, make sure you have a full set of steel armor and some fully upgraded alloy greaves or you are going to get your ass kicked, Guard +2 seems to be a must.
