No there are not certain advantages solo'ing. There is nothing stopping you maximizing a single character* in a party of four. The advantage over soloing is 3 extra player actions (attacks) and 3 more targets for the enemy to focus on.
*Who does'nt maximize the player character?
I've done solo missions with Astarion the assassin where the other characters would just get in the way. Likewise, when exploring you often have 1-2 characters who are tag-along liabilities.
I for one don't maximize any one character, I optimize the party balanced against ease-of-play.
Back to Luality, look at her action economy, she's got two standard actions in that fight. Effectively she's got two high DPR characters, which in my view undercuts the argument she's truly solo'ing.
And there's part of your answer to higher difficulty; either limit haste in all its forms - or generously hand it out to most NPCs.
