can beat Will's Exeggcutor with Flying attacks, but struggles against the rest of hs team. It can handle all of Koga's Pokémon with Psychic attacks except Forretress – watch out for Explosion. If fast enough, Xatu can defeat all of Bruno's Pokémon. Watch out for Hitmonchan, who has Thunder and Ice Punch, as well as Machamp with Rock Slide. Onix can be taken out by a Psychic attack if you're a high enough level, but I would suggest using Victreebel. Karen is a problem for Xatu, but it can beat Vileplume and Gengar (if you're fast enough). Lance presents a problem. Gyarados knows Ice Fang, Aerodactyl knows Rock Slide, Charizard knows Shadow Claw and two of his Dragonite know Blizzard and Thunder.
Xatu is the superior choice. She has a good matchup against most of the E4. She can also cover most of your team's weaknesses. Try and get her the TM Shadow Ball so that she can take on the Psychic E4 member.
Good luck, Nuzlocker!
*Lucky. Your Victreebel lived ;-;