The Boss List

1.  Shattered Lancer
2.  Shattered Susie
3.  Shattered Ralsei
4.  Shattered Kris
5.  Shattered Bone Duo (Sans and Papyrus)
6.  Error!Sans 1
7.  Shattered Fire Trio (Toriel, Asgore, and Asriel)
8.  Error!Sans 2
9.  Shattered Ink!Sans
10.  Shattered Alphys
11.  Shattered Undyne
12.  Shattered Omega Flowey
13.  Shattered Swap Brothers (Blueberry/Underswap Sans and Carrot/Underswap Papyrus)
14.  Shattered Mettaton
15. Soulshattered 1
16.  Error!Sans 3
17.  Soulshattered 2
18.  Shattered Riverperson
19.  Shattered Mettaton Neo (This time he has more HP and has attacks, he's also immune to slashes, so probably find something)
20.  Ultra Soulshattered Phase 1
21.  Ultra Soulshattered Phase 2
22.  Ultra Soulshattered Phase 3

You get the Shattered Person back to normal after their Battle, so you can gather a team to stop Soulshattered.

Secret Boss: Shattered Power Skeleton Trio (Power!Sans, Power!Papyrus, Power!Gaster)