According to someone I know on Discord (I didn't watch it btw since I was out and about), it went like this:
"Fern has wings, he emerges, possibly part of the rising and grass octopus, kind of like a pterodactyl Fern
some buildings on fire, not sure if a Marcy/Simon flashback
something resembling the dark cloud attacks that mutated monster, Marceline's doing?
Finn's arm is back to a stump...what happens to bionic arm
him/Jake/PB get blasted back by force of something
"they're going to be talking about this fight for years" Jake goes blue alien mode"
So yeah, that's pretty much it.