It's possible to reflag for Shadow Crypt in around 10 minutes on any character. After you collect the end reward from the Herald, so that Shadow Crypt shows as completed, abandon any one of the four flagging quests, re-acquire it, run it and turn it in. Now go talk to the Herald again to get Shadow Crypt bestowed. He will actually say that this will bestow Shadow Crypt without resetting any other quests, so it's reasonable to assume this is WAI.
The fastest necro2 quest by a decent margin is Tomb of the Shadow Lord, using the solo technique. This requires using a hireling (or a pet), even if you aren't soloing. You need a hireling to open the second gate pair to let the rest of the party in the north side. No exotic abilities like dimension door are required.
Here's a video from start to finish, though fair warning that I'm not particularly good at this quest. I run it on hard in the video because I already ran it elite for bravery the first time I flagged for Shadow Crypt: