Jared Knabenbauer

Alternatively, a Young/slightly younger AdultGreen Dragon is seeking territory to call his own. It so happens to be this forest. The Barb runs into the dragon. Death ensues.

This is why I or my GM never awards individual experience, but party exp. In games like Rogue Trader, players can do individual things on the ship. But it is usually beneficial for the group.

Any long range cantrip Eldritch Blast/Firebolt paired with Misty Step as a bonus action so the spell caster can teleport and move even more after casting. 2+ enemies with that running around him should be enough to scare him back to the group. They return together and win!

yeah, I don't think you'll find anyone trying to "grind" in D&D unless there's some crazy meta gaming going on. Perhaps the DM @GuyGorby should audit his xp awarding

this sounds so terrifyingly ominous out of context

Harsh but fair. Especially in a a GROUP based role-playing game. He may not be playing wrong, but sounds like he's playing in the wrong group. Sounds like your stereotypical munchkin.

-- Rust Monster?

Force him into a situation where his weaknesses are played up, make him realize that he needs the rest of the party. Could put him in a situation to nullify his magics for a bit perhaps and make him have to think outside the box.

Have a creature challenge him to a solo fight; turns out it's a stupidly powerful mob that one-shots him.

How deliciously evil!

Yeah, make sure he doesn't overcome the challenge or else he will think that being alone is beneficial

Basilisks with a feral druid. Baddabing.