0197 Umbreon

Snarl is recommended over Feint Attack for its very fast energy generation, though the latter does offer more damage output. 

Foul Play is usually the preferred move, due to its lesser energy cost, but Dark Pulse does provide some extra power for one or two more fast moves needed to charge it. Umbreon's lack of coverage moves is its liability, so while Community Day-exclusive Last Resort does cost more energy to use and is Super Effective against nothing, it's a must to deal neutral damage if the opponent would resist Dark-type moves. Psychic is an alternative coverage option added during the 2nd Eevee Community Day, and is preferable to retaliate against Fighting-type Pokemon, but risks Umbreon being completely resisted by other Dark-type Pokemon. 

This is a very specific set that requires at least one Elite Charged TM. It combines the perks of both of Umbreon's coverage options against its vulnerabilities versus Fighting-types and opposing Dark-type Pokemon. However without STAB Dark-type moves, Umbreon loses a lot of damage output against neutral opponents, and is also completely walled by common Steel-types. 

Umbreon is a very strong Pokemon in Great League, as its low ATK stat distribution gives it extraordinary bulk. Its relatively neutral Dark typing makes it highly viable in a variety of team compositions, making it a great choice overall. Umbreon's main struggles are against the popular Azumarill, other Fairy-type "Charmers," and Fighting-types. The addition of Last Resort and Psychic following the first and second Eevee Community Days, respectively, are valuable coverage options for Umbreon, but not enough overcome its weaknesses in a straight-up fight. 

With the advent of Level 50, Umbreon can now reach 2445 CP with Best Buddy boost. This brings it from "just some random Dark-type" to an insanely bulky monster, with a stat product significantly surpassing that of Giratina-Altered. It has positive matchups against Pokémon like Cresselia and Giratina in Open Ultra League. However, it mostly shines in Premier Cup, beating a meaningful portion of the metagame off bulk and charged move spam alone.

Master League isn't meant for Umbreon unfortunately.