For single battles, Leafeon is excellent. For double battles, Leafeon is a bit more limited, but can work some things out/
If it is to be used with another evolution of Eevee, he can be used as support to team up with, for example, Flareon.
With moves like:
And IVs focusing in HP, and EVs in HP and special def, for making good use of his already higher defense stat in that particular role.
On top of boosting Flareon's Fire type moves, Sunny Day also make good use of both of Leafeon's abilities. Also, Leafeon's Leaf Blade covers all Flareon's weakness.