On the latest PTR it looks like the undying achievement (and title) can be done in multiple raid resets now...meaning if you beat, say, heigan one week with no deaths, itll be checked off, and you just have to do each boss that way until they are all checked off...
the catch is that a wipe counts as a death, so you have to not only have zero daeths on a boss kill, but also do it on the first attempt...
Honestly, I think even now this achievement is completely do-able for us. We're already able to one-shot the Spider wing without a problem or death, and generally in 10-man it's only the bosses in the abomination wing that give us any sort of trouble. 25-man's a bit harder with the lag issues, but I think we could manage it ^^