In 18 BBY some of the Jedi confirmed to have survived Order 66 so far are able to gather together and decide to take on the Sith and their new servants:
Jedi survivors team:
1) Yoda
2) Obi Wan Kenobi
3) Quinlan Vos
4) Kirak Infila
5) Jocasta Nu
6) Barr(no “execute order 66” trick allowed for him)
VS.
Empire team:
1) Emperor Palpatine
2) Darth Vader
3) The Inquisitorius
All in their canon versions. Vader is as strong as he is in the latest Darth Vader issues ( which are set around 18 BBY)
Vader isn't gonna be much help here, at this point Obi-Wan takes him pretty easily, all inquisitors are quickly beaten, then the Jedi dogpile on Sidious.
Sidious attention will be focused on Yoda, he can't focus on anyone else, especially not Obi-Wan, since he's not casually killing Obi-Wan Kenobi while he's in Soresu mode, it's not happening.
First, Yoda vs. Sidious is a long and good fight.
Meanwhile Obi-Wan destroys Vader.
The other 4 Jedi are in a good fight against Inquisitors, but Quinlan tips the scale.
Then: Obi-Wan aids other 4, and Inquisitors are quickly destroyed.