A Lvl 3 Wildfire Druid/Lvl 3 Assassin Rogue can drop 24d6 at the start of combat by combining:

Bugbear's Surprise Attack

Wildfire Druid's Scorching Ray

Assassin Rogue's Assassinate

Each ray should deal about 8d6

Credit to D&D Daily

It should be noted that this only works with the 2014 Assassin subclass, and only if the target is surprised, which is necessary for the Assassinate ability to turn normal hits into critical hits. Otherwise it's only 4d6 per ray (2d6 from the spell itself and 2d6 from the Bugbear ability) though that's certainly not nothing.

Also, the Wildfire subclass is not really contributing anything to this; anyone who can cast Scorching Ray can do the same thing when combined with the Bugbear and Assassin abilities like this.

Quote from wagnarokkr >>

It should be noted that this only works with the 2014 Assassin subclass, and only if the target is surprised, which is necessary for the Assassinate ability to turn normal hits into critical hits. Otherwise it's only 4d6 per ray (2d6 from the spell itself and 2d6 from the Bugbear ability) though that's certainly not nothing.

Also, the Wildfire subclass is not really contributing anything to this; anyone who can cast Scorching Ray can do the same thing when combined with the Bugbear and Assassin abilities like this.

 All correct! Wildfire Druid for flavor and the Wildfire Spirit is great utility. I also just like the subclass.

Edit: REMEMBER CONJURE MINOR ELEMENTALS! This adds 12d8 elemental damage.

Yeah, it's a fun one, especially if you like fire

Quote from wagnarokkr >>

Yeah, it's a fun one, especially if you like fire

 Yup. It also gets stronger if you continue to level up in Druid.

9th Lvl Conjure Minor Elementals + 8th Lvl Scorching Ray + Bugbear's Surprise Attack + Assassinate can deal up to 80d6 + 72d8 fire damage. Insane

I highly doubt a DM would let you pair 2014 versions of subclasses with 2024 versions of spells.

The chance is small, but still there