I made to the E4 losing only two in Black version: a fodderish Lillipup I had to sacrifice to a Bide-spamming Timburr in the office building in Castelia, and (sigh) my Level 45 Excadrill while training her on the route just before Victory Road. Turns out a single Metal Claw just barely fails to OHKO a wild Sawk, and those things know Counter.
Granted that was my second playthrough after a disastrous first run where I reset around thirty times, but I was still pretty proud of myself for making it that far with virtually my entire team intact. The trick is mostly just to be very, very careful. Overlevelling, especially before taking on Lenora, is absolutely essential too.
(And in case you're wondering, the last bit of my run didn't quite go down so well. I beat the E4 and caught Reshiram without losing anyone, lost only my Krookodle to N, and proceeded to get all but my Samurott OHKOed by Ghetsis' Hydreigon. That thing is an absolute MONSTER. I managed to win in the end, but it was close. Very close.)