if you have enough game knowledge, many things are possible. There are streamers who full clear the game solo without multiclassing, cheesing or using the forbidden dark art of barrelmancy. You just need to know which fights to avoid until certain breakpoints, when to flee combat, how to skirmish and kite with the mobile feat, how to quickly get your build online, etc. You also use shovel (the little demon, not the tool) a lot to trigger fights so you don't get surprised.

For example, you can get to level 4 without any combat whatsoever after the tutorial. Just by using enhanced leap and featherfall and jumping around doing dialogue checks disguised as drow. Level 5 is the major power spike where the player becomes a lot stronger (Level 3 spells and Extra Attack). Once you're 5 you can basically solo all honor mode combat with the proper gear and elixirs and tactics.

There are a couple of mandatory fights you can't flee from (Act 2 boss has one in particular). You do need to prepare for Myrkul with Doom Hammer, darkness arrows or spells, lots of grenades, lots of speed pots, etc. You also need crit immunity (adamantine shield or armor for the most part), a death ward/cheat death effect (half-orc is the meta but not required) and complete knowledge of everything that can paralyze you (fuck hold person, gnome is the 2nd best in the meta for this reason). Those things are game over at some point if you don't have a way to prevent it or get ahead of it.