Well, it is my favorite topic
The most difficult party is the party you do not know how to play. Period.
Want some proof?
On the core rules it is possible to beat the game solo with ANY class if you know how to play it. This part is proven.
Now, additional party member is again a character who might have been your proagonist copy (well, not exactly, but still pretty close). So no matter whom you add, it will make your party stronger, right?
If you want some more challenge, you can change your difficulty setting to higher, or play with incomplete party (1-5 members), or keep your party around but do not use them (although this sounds a bit silly to me).
Good luck,
Mishik
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