I just reset my Focus via Zune. I did it with the disconnect method, waiting about 3/4 seconds. Took a couple tries, but did the trick (yesterday I tried 20 times with no luck, today it was a success at the 3rd attempt). I went to 8779 first, then 8783 and finally
8858 (had to do the disconnect for each of them). Oh, also, I took the SIM out while I was updating, just in case (so the phone doesn't know I'm using the T-Mobile network, just in case that made things more difficult).
Note that my Focus is the AT&T model but I flashed it long ago with the original retail Rogers ROM (which is practically stock WP original rom) because I hated all the stupid personalizations made by AT&T, so my Focus thinks it's an i917R instead of an i917 - this could be why I was able to do the disconnect thing. In any case, I very highly recommend you ditch the AT&T rom in favor of the clean Rogers one, it's the perfectly clean, non-homebrew original used by Rogers in Canada, so you will get all updates through Zune perfectly normally.
Update: after about an hour of use, this 7.8 update is quite noticeably speedy. Better than 8773 was for my Focus, that's for sure. The new emerald theme also looks GREAT in the AMOLED screen!