Keep in mind that this guide will need you to know:
a. Basic hex editing (use HxD or another hex editor that works for you)
b. Patience to read
c. ALWAYS keep a safe backup of the unmodified save before proceeding
d. Able to understand page with offsets, and refer to the correct game
Which in point (d)'s case, will refer to here:
Gen 6: https://github.com/kwsch/PKHeX/blob/b5b4b67263e905ad4ba34ae574c68d20714d2047/PKHeX.Core/Saves/SAV6.cs
0. Remember to keep a safe backup of your save, before proceeding.
1. Load up your save, and a working donor save, in hex editor
2. Grab Situation and FieldMoveModelSave from the donor save, and paste it into your save.
3. Once you took those portions from a donor save,
load up the edited save in PKHeX
4. Export the edited save from PKHeX.
(This fixes any checksums.)
5. Now test the save on your 3DS.
6. If save doesn't work, commence hex editing via block import again.
This time, copy MyStatus block from a donor save.
7. Perform checksum fixing via PKHeX [step (4)]
8. Load up this new save on 3DS, to confirm if it works.
9. If it does, go ahead and edit the trainer details (OT Name, Trainer ID, Secret ID) to match your old save on PKHeX.
Strictly speaking, if you understand the importing steps above, you can import in other blocks