I stalled Snorlax with a Level 16 Gastly using Curse.
Venusaur only has Grass moves, which are resisted by several types. Hell, a Sunny Day actually increases its Fire weakness, aiding its own demise. Or as you said, Fly can avoid his SolarBeams.
Espeon I managed to beat with Houndour. Houndour came a little late at Kanto and underleveled but after a little babying he was decent. He's also pretty good at grinding at Sabrina's gym lol.