Help with 6th member

[RoloMaisso]

Hi, I'm stuck at home and decided to replay Pokemon Emerald.
As the title suggests I need help with the last team member.
I currently earned my 7th badge.
My current team is as follows:

Marshstomp - Relaxed (lvl 50)
- Ice Beam
- Mud Shot
- Rollout
- Surf

Gardevoir - Quirky (lvl 50)
- Confusion
- Psychic
- Shadow Ball
- Calm Mind

Slaking - Timid (lvl 50)
- Return
- Yawn
- Strength
- Hyper Beam

Raichu - Mild (lvl 50)
- Thunder
- Thunderbolt
- Slam
- Thunderwave

Altaria - Adamant (lvl 50)
- Aerial Ace
- Dragonbreath
- Dragon Dance
- Flamethrower

For what I understand I think I pretty much cover all possible weaknesses. Though I might be wrong.
I don't know of this helps, but all my pokemon have Pokerus.
I'm not worried about Elite Four or final showdown with Steve, but Battle Frontier seems tough. I mostly need help with final member to cover for any weaknesses.
I been using an hm slave for fly, rock smash, etc.
Any suggestions for a final member.

Thanks and stay safe.

 

[Ditto B1tch]

You have nothing against bulky normal types (Snorlax, Blissey, for example), so I would recommend a fighting type