Han Solo killed the Emperor, for example. Granted, Palpatine was in a severely weakened state, but nevertheless.
Then again, endgame story content is a group
content for a very good reason. Even some of the strongest Force users in the galaxy (namely Emperor's Wrath, Darth Nox, Hero of Tython etc) couldn't deal with the Dread Masters/Soa or even an extremely powerful Hutt crimelord without little help from their allies.
As for the soloable class/faction based stories, if I remember correctly, you get to smoke a few powerful Force sensitive adversaries as a non-force sensitive class.
All in all, one cannot come out as victor in a war without any external help. Rebel Alliance wouldn't have defeated the Emperor without Luke and I somehow doubt Luke would've had any chance to prevail against the Empire all alone.