How do you manage the Fights in the Underdark with a solo Druid?
Go to YouTube, type "sin tee" in search bar and watch his most recent video. It's super easy especially if you prepare before the fights.
The simpler answer to this sentence is "play like a fighter, exploit dual wield" and before you get conjure flame blade, just play longbow wood elf with moonbeam in the meantime.
You could go through sin tee's videos and they're all the same thing. Either abuse flame axe+ offhand with fire dip for huge burst damage from dual wield, play a caster with fire ray helm and force missile necklace, or a battlemaster 2H build. Doesn't matter the class, basically plays the same because the balance of actual class skills, and particularly how worthless most spells outside force missile/fire ray/inflict wounds are, and the fact the only utility spell you get to use courtesy of the trash Concentration mechanic is Bless because Bless makes a huge different in landing non-martial attacks.
The spell casting in this game is totally horrendous gameplay. Even the melee is not that much more interesting even if it's supremely better.
Divinity by far had a far better repertoire of spells and combat. The sheer variety of melee attacks and spell combos isn't even close compared to this snoozefest of "single ability per turn that, if not a melee backstab, may probably miss 40-50% of the time" is among the worst design decisions in place.