Athenodoros is right. For an in-game team, you don't really need to worry about EV training or even getting the right natures (I usually just try to make sure I don't get horrendous ones, like an Adamant Alakazam). You'll usually want a team filled with sweepers to allow you to progress through the game as fast as possible. However, Umbreon's stats and movepool make it practically impossible to use him a sweeper, so you'll have to go a bit of a different route with him. Since you'll likely be fighting several battles in a row without healing, you'll probably want to avoid low-PP moves like Fire Blast/Eruption/Overheat on Typhlosion or Leaf Storm on Victreebel. If you want an example, I'd suggest something like this:
Typhlosion
-Flamethrower
-Earthquake
-Thunderpunch
-Rock Slide
Rock Slide is mainly a filler, but it's your team's best offensive option against Lance, and I assume you plan on fighting the Elite Four a lot to level up to 100.
Victreebel
-Sludge Bomb
-Seed Bomb / Energy Ball
-Sleep Powder
-Synthesis
Umbreon
-Dark Pulse
-Return / Psychic
-Confuse Ray / Toxic
-Moonlight
Something like that should be fine. You can pretty much pick whatever you want for items, although you probably want to stay away from Life Orb and Choice items. Items like Charcoal and Wise Glasses work fine, and are usually what I use in-game. Leftovers is good too, although you may be annoyed at watching the healing animation every turn if you're not at full health.
Good luck!
