First-time DM here, with mostly first-time players. I'm playing somewhat of a mix between the original LMoP and some of Matthew Perkins advice in between.

The party has just entered the Redbrands hideout, and I'm prepping the next session.

Matthew suggests having Glasstaff come out when the fight is down to the last redbrand ruffian, and use his scroll of fireball before running. (Link to his video here)

I really like that idea from a storytelling point of view, but I don't think any Lvl. 2 party is able to survive that for sure, right? Fireball does 8d6 fire damage, which is a lot. Even my fighter only has 20 hitpoints, the rogue and cleric obviously less. And they're probably not at full HP after the Nothic.

Having a successful fight followed by an unexpected wizard barging in for a total party kill doesn't sound like fun. I considered burning hands, but even that at 3d6 could potentially be a TPK.

I don't really like the idea of Glasstaff fleeing quietly, or giving up without a fight at all. What makes sense to do here?

One option could be to debuff fireball; making it only 2d6 damage, or whatever works at the HP they have when they get there. But I'm not sure if my players will appreciate me nerfing the big boss.

Any suggestions, thougths, comments, insults?