They would start by (hopefully) finding out what the lion is.

(I mean, half the time they charge in half-cocked if people are in direct danger, but they prefer to do their research first).

First would be to get innocents out of the way. At a zoo, they might end up helping the zookeepers get some of the animals out of the way so they don’t lose any of them, especially those endangered species.

Just gotta save the girl the lion took as a hostage to lure the hunters to it.

Finding something to kill it with would be work, since it can’t die by any mortal weapons. Angel blades would be easiest, if they knew any angels at the time. Definitely not a mortal weapon, and almost nothing can stand up to it.

Dean might be tough, but he’s not as strong as Heracles, so he wouldn’t be able to strangle the lion if an angel blade isn’t an option. His best option would be shooting it in the mouth, if he figures out its weakness.

Monster gets the gank, hero gets the girl. Sam rolls his eyes.