I would say a couple different things.
1) an HP tank. Snorlax, Lapras, or Chansey work well here if you have them.
2) a different type. You have a bunch represented, but not a Rock/Flying (Aerodactyl) or a Fighting (Hitmon-) Obviously, others you can use as well. Steelix is resistant to a lot of Elite 4 attacks... But if it's your first time, you can't get that yet...
May I suggest a Psychic? Like a Hypno or Alakazam as well... Although with Exeggutor, this isn't entirely necessary.
You could also do a Flying Poke. Fly works well for avoiding an extra turn...
Hope this helps!
It's great to complete a Pokedex! So I'll need some help...
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