Xenoblade: Tephra Skip Guide

Perhaps one of Xenoblade's best-known glitches is the Tephra skip, an elaborate exploit that requires a lot of patience but allows you to reach level 50 within the first couple hours of the game. It's not infallible—while there's no cap on experience gained at a time, there is for Art and Skill Points, so you're significantly handicapping yourself on that front because of the way reward scaling works against underleveled monsters—but it makes for a neat party trick. Here's a step-by-step guide to pulling it off.

To do this, you're gonna need to rely on the random chance to block an attack. The absolute highest block rate you can get this early on is Reyn's Block Guarder at 6%, which you can easily obtain by trading with Mefimefi or overtrading with Kenny Rohan. You want to be at least level 8, as yellow difficulty will disadvantage your block chance against the level 10 enemy you're "fighting", but not so high-leveled that one person can kill it alone.

Mefimefi's basic trade table. The Block Guarder, her only item, is highlighted.

Make your way on into Tephra Cave until you get to the locked door. Instead of walking past like we're supposed to, we're gonna just make our way right on through it.
To do this, shove one of your other party members against the right corner of the door, and then just cram yourself between the two to the best of your abilities. You will probably have to spend a good long time on this part, because their pathing will make this pretty difficult as they politely walk out of your way to make room for you, but if you can get the angle just right you'll end up clipping straight through to the other side.

The locked door in Tephra Cave. Reyn is positioned in the right corner, and Shulk, controlled by the player, stands behind him. Arrows indicate the boys' roles and the ideal angle to squeeze yourself between Reyn and the door.

Continue onwards all the way to Vilia Lake. You should probably save your game here! Climb up the vines and run towards the slightly too-tall ledge at the end of the path.
now it's reyn time
Put Reyn in the lead with the Block Guarder equipped and remove all of Shulk and Fiora's armor. Their purpose in life is curently "bait".

The party menu, showing the ideal setup. Reyn is in the lead with the Block Guarder equipped, and Shulk and Fiora are without armor and in their underwear.

Target the Cave Skeeter flying above the lake and engage it in battle. (You can also use the "lure" command, which will cut down on waiting, but it's a lot less reliable that it'll be in range for it.) From here, you simply have to shove one of your beloved friends off the cliff to go fight it by themself and die about it; this is easier said than done because there's a system in place that prevents characters from falling off ledges, but you can sometimes finagle their pathing to run straight off on their own, which can generally be done most consistently using the Battle Commands.

Reyn overlooking the lake from above. Shulk, nearly dead, is fighting the Cave Skeeter on his own far below.

Your very brave friend will fight the bug on their own and lose, and then (assuming it hasn't strayed far enough away) it will switch its aggro to the next party member and fly up to attack you next. This is the part where you select "flee" and make a break for the ledge.

Reyn running into the ledge with the Cave Skeeter preparing an attack behind him.

At the very end of the path, just keep running straight into the ledge while the Skeeter attacks you; time your jumps against its attacks to get hit at the peak of your jump. If you randomly block an attack, the extra knockback will be just enough to bump you onto the ledge! (This will, in all likelihood, take a Very Long Time and many, many deaths.)
Because this area is only supposed to open up during the endgame, none of the very scary enemy spawn flags are active yet; it's completely empty, so you're totally free to run through unaccosted, collect experience for visiting all the locations and landmarks, and become wildly, disgustingly overpowered! Congratulations!

Reyn discovers the Heavenly Window, an empty endgame area. The party has recieved 165160 EXP each, bumping them from level 8 all the way up to level 43.

From what I've heard, this skip has been fixed in Definitive Edition... which also unintentionally added a way easier way to get the same effect. I have yet to try it, but if I ever do it'll be right here on this page.

Xenoblade: The Tephra Cave Exploit