Playing the game with an underleveled team

Best way to kill any Umbreons is to Taunt it, set up a Sub, and just spam Focus Punch like crazy. It's attack stats (both) are so feeble that it shouldn't be able to break your Sub. You could always try using Reversal too. Not sure if at the point in the game you're at, if you could get something like Scyther with Endure + Reversal or not. Should be able to though.