Well. I've cleared out entire rooms with naught but a not-so-cheesy regular assassin/gloomstalker/fighter no problem. If you have movement speed and the ability to hide, you can solo pretty much the entire game without too much hassle. Most 'hard; fights have environmental advantages that can be exploited, multiple avenues of attack and escape and so forth. And that's *without* using the cheese builds.
The game has *so* many options for you to experiment, that yes, it is certainly doable solo on each class.
And, true, 5E doesn't let you stack multiple protections all at once because of the Concentration mechanic, but in all fairness, I like it better this way. High level Mage Fights in BG2 are the most boring ever, and you always had to pack your spellbook all the same obligatory spells like Spell Immunity Divination and Ruby Rays of Reversal just for that. You'd have maybe one or two spells that actually did something fun, the rest was protections and dispels.
