I spent a couple months creating a single large shelveset. I shelved it a couple months ago. Now we want to merge it into the mainline.
The base of the shelveset is four months old, and the mainline has marched forward during that time.
Is there a known strategy for dealing with this mess?
Find a point in time from "a couple of months" ago and create a branch from that. Unshelve the changes into the branch. Commit the changes, then merge your branch back into the trunk. You may have to do a baseless merge; I'm not sure off the top of my head.
In the future, don't use shelvesets for this since they are not intended to be used as a replacement for branches. Shelvesets are intended to allow you to do short-term suspension of work in progress.