What should my final team member be for the Pokemon League? (Sapphire)?

Question

So I'm about to head into the Pokemon league in my Nuzlocke playthrough of Pokemon Sapphire. The fact that this is a nuzlocke means that I can't really try to catch a specific Pokemon to put onto my team: I can only choose someone from the box to replace my HM slave.

My Team:
-Swampert
-Swellow
-Sableye
-Ludicolo
-Hariyama
-Zigzagoon (HM slave)

The options I have in my box are:
Silcoon, Beautifly, Tentacool, Roselia, Numel, Wingull, Spinda, Seviper, Zubat, Sandshrew, Wynaut, Lileep, Electrike, Gyarados, Magnemite, Kecleon, Oddish, Castform, Machoke, Shuppet, Natu, Girafarig, Psyduck, Doduo, Vulpix, Clamperl, Spheal and Lairon.

Any of these any good against the elite four?

Answer 1

The Pokemon you already have on your team cover all types bu Flying and Dragon supereffectively by their STAB attacks, both of which can be covered by an Ice-type. Therefore, evolving Spheal into Walrein would likely be your best bet. However, this would mean three Water-types on your team. If you don't want so many Water-types, you could instead try to use Castform Snowy Form for Ice coverage.

TL;DR: Use Walrein or Castform Snowy Form, both with some sort of Ice attack.

Hope I helped!

Edit: You said you already have Ice Beam users, so use Camerupt to cover the weaknesses of your Water-types.

Comments

There are not that many grass or electric attacks in Ever Grande, so using 3 waters is very reasonable as long as you look up their movesets on Bulbapedia to make sure you don't get surprised. Don't use Castform. Its stats are bad.
Sableye's stats are also kind of low. Depending on how high its level is, you might want to either level grind Sableye some more or replace it with Magnemite.

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Alternatively, you could teach one of your existing team members Ice Beam, and use something like Camerupt to cover your Water-types.

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Ice beam Swampert is pretty strong. You can get more ice beams at the Mauville Game Corner if your ice beam user dies.

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I have 2 ICe-BEam users already.

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Then you're probably fine.