Just make finn the child of destiny or something.
Lich is just a powerful magus who wants to black out the stars to make the coming of Golb possible.
As long as there is light and life and hope in this universe there is no place for Golb.
Give him a magical techno arm after he sacrafices it to save his godson sweet pea.
Give him billies sword or at least have it so that he can summon some kind of magical light sword out of nothing.
Have him use everything he's learned and gained up to that moment from adventure time.
His magic powers, his plant based arm abilties his fighting skills even all of the cool knick knacks that he's had. Have him use everything to stop some mid boss before Golb and seal it off back into it's dimension.
Have his friends that he's made along the way fight off the other disciples of golb.
Have simon "sacrifice" himself to save betty but wakes up completely ignorant of what has happened for the last few centuries but have some kind of wierd blue skin and ice breath.
Gunther is the new queen of ice
PB and Marcy get their happy clam jousting end, Finn wishes them good luck and leaves them to their life. Basically stating that his time with them is over.
Golb, in a single moment of spite, releases an evil power upon ooo and across the stars.
The last scene is Finn and Jake standing up and going after the dark monster and hinting that they'll find all of those other monsters out there.