Minimal weight lifting on a bench and some dumbbells, steadily increasing, no real set goal just keep pushing yourself to do more and do better than the last.
Final bosses should be memorable. Take DS3's Soul of Cinder, was memorable mainly due to the nostalgia of the music. Arguably Pontiff was a more memorable fight for me but was plucking strings when the music of the second phase of Soul of Cinder kicked in. Dependent upon the game too.
"They asked me how well I understood theoretical physics. I said I had a theoretical degree in physics. They said welcome aboard."
-Mr. Fantastic, 2281, Fallout: New Vegas
PayDay 2 and Dark Souls 3, though it feels like Fallout: New Vegas has spent more of time than the others combined, probably due to offline mode, mods, and some console play rather than PC.