Ideally, you'd want to get Espeon's hidden ability: Magic Bounce. In game however, where hidden abilties are hard to access, Synchronize is your only option. Since you seem to be willing to both teach it the tutor move Signal Beam and EV train it, I think it would be wise to breed a dream world eevee just for use in battle.
As for the set, Espeon tends to run a more support oriented set with Dual Screens or a Baton Pass set, as exclusively offensive sets are better done by other psychic types like Alakazam. However, a fully offensive set can take advantage of a Life Orb or Choice Specs to boost power, and with Espeon. I'd personally go with Choice Specs as with Life Orb espeon would be worn down quickly. If you worry more about Espeon's ability to survive rather than how hard it hits, I'd give it a Focus Sash to allow it to survive at least one hit as it is rather frail.
