If you're going solo, then bardic inspiration wont work either.
Which class are you using thorn whip from?
I am planning on playing a fighter and getting thorn whip from the Spell Sniper feat. I am not going solo, so bardic inspiration is a valid suggestion!
For clarity, bardic inspiration is a bonus action by another character that you can then use on your turn. Lore bards can use cutting words as a reaction, bot that wont give you an attack bonus.
If you are okay with having a bard set you up on their turn, then a Faerie Fire (action) and Bardic inspiration (bonus action) combo would set you up perfectly. And it would make cleaning up what's left of them easier, too.
