All right - "finished" my solo run with Sebastian the Stoutland through White.
Original post
So - after already beating the entire E4 AND not one, but TWO additional high-level trainers, sitting through the closing credits and even seeing the game tell me "The End," I slogged through pretty much the entire post-game so that Sebastian could level up far enough to at least compete against the level 70+ pokemon he was going to have to face when he RE-challenged the E4. So we went all the way around all the rest of the map and through every single post-game dungeon, cave and route and fought every single post-game trainer, then when we had completed all of the content in the entire game, we went back to the E4, so that Sebastian could beat them for the second time and beat the "champion" (you know - the guy who lost to the guy we already beat the last time).
Sebastian was level 85 going in, with Return, Crunch, Ice Fang and Dig.
Shauntal was okay. Sebastian got Mummy off the Cofagrigus right off the bat, but since his regular ability is Intimidate, it was no big loss, and it was sort of amusing to pass Mummy back to the rest of her team. Jellicent was a bit of a hassle, since its Cursed Body disabled Sebastian's Crunch, but then Drifblim came out next, so Ice Fang was fine, and by the time Chandelure came out, Crunch was working again. Sebastian got burnt by its Flame Body, but it was already far enough down that he KOed it with the next hit anyway.
Grimsley went down pretty easy - mostly Returns and a Dig for the Bisharp.
Caitlin was a bit of a challenge. Musharna and Reuniclus were both bulky enough to survive a Crunch and hit pretty hard in return, and Metagross could've been trouble. I figured it had Earthquake, which made Dig too much of a risk, so I opened with Crunch, which only took about 1/3 of its HP. But then it hit with Giga Impact, which took Sebastian down to a bit less than half, and that was my chance. Since it had to recharge on the next turn, Sebastian used Dig and KOed it before it got the chance to do anything else. Then it was just a matter of chipping Bronzong down with Crunches and that was that.
Marshal was sort of tough. Breloom went down to three Ice Fangs (with a Full Restore after the first one) and got in a Sky Uppercut along the way that knocked Sebastian down to about 1/2. I took a chance and opened against Mienshao with a Return and 1HKOed it. Then Sawk came out. Luckily I decided to give Sebastian a Hyper Potion and got him to 100%, because Sawk's Close Combat took him all the way down to 28 HP (from his then-max of 275). And that was the way the battle went - I'd heal Sebastian, then Sawk would hit for almost all of his HP, then I'd heal him again. I finally got in one shot when he missed a Stone Edge and went for an Ice Fang, hoping for Frz, but no such luck. But a few turns later, he missed with another Stone Edge and Sebastian took the opportunity to KO him with a Return. Then Throh came in, and wasn't as much of a challenge as I was afraid he'd be - he went down to two Returns and only hit Sebastian with a pretty feeble Retaliate. Then Conkeldurr came in - Sebastian opened with Return for a bit more than half of its total and Conkeldurr countered with Hammer Arm, which knocked him down to about 100 points. Another Return and it was over. And lastly Toxicroak. I took the first turn to heal Sebastian and Toxicroak hit him with a Toxic. Dig on the second turn and Toxicroak missed with a Venoshock. Then the Dig hit on the third turn for a 1HKO.
Then we went and fought the guy who had already lost to N the last time through, and mopped the floor with him. Accelgor went down to a single Return (it went first, but missed with a Focus Blast). Druddigon took three Ice Fangs (the first one knocked it into the yellow, but then the second somehow failed to take the less-than-half that was left, so we had to mop up with a third one). We had to chip away at Escavalier with Returns and Digs to take it down, but it was never a threat. Bouffalant was more of a challenge, but it was mostly just a matter of healing off the Head Charge damage and waiting for the recoil to faint it, with one Return along the way. I took the first turn on Volcarona to heal Sebastian, and a good thing, since it opened with Overheat and knocked him back into the red, but that was it. It got in one Quiver Dance while Sebastian took it down with two Digs. And Vanilluxe went down to a single Return.
And in case I didn't make it clear enough along the way, I just want to hammer on this point - the rule that says that a challenge on Black or White isn't complete until you RE-challenge the E4 is poorly considered and, IMO, should be eliminated. I finished this challenge in part because I didn't like leaving it unfinished, but mostly just so that I could write this post. I really think that that rule should be removed, as all it really accomplishes is that it makes the requirements for "completing" Black and White MUCH higher than they are for any other games in the entire franchise. I could see it if the first time through the E4 was easier than it is in the other games, but it's not. It's pretty much the same as any other E4 challenge, and if anything it's arguably a bit more difficult, because once you're done with the E4, you don't just have to only beat one high leveled trainer - you have to beat TWO. That should be sufficient, particularly since the game itself treats that as "The End." There's simply no justification for having to beat the E4 TWICE to "complete" the game.