Trying Out Different Team

[Death]

Okay, so I put together a team on Pokemon Showdown on the fly to battle a friend of mine. This team wasn't really well thought out, but I was wondering if there is a way to make it more viable?

I'm okay with swapping out Pokemon/move sets, but I want Trevenant to be permanent on this team.

I'm on the fence with Magnezone, I never liked breeding Pokemon for Hidden Power, but so far it's been pretty helpful trapping other steel type Pokemon (Klefki, Skarmory, etc.).

Current Team
Skarmory @ Leftovers
Ability: Sturdy
EVs: 224 HP / 252 Def / 32 SDef
Impish Nature
- Whirlwind
- Stealth Rock
- Brave Bird
- Roost

Tentacruel @ Black Sludge
Ability: Rain Dish
EVs: 248 HP / 244 Def / 16 Spd
Timid Nature
- Rapid Spin
- Toxic
- Scald
- Protect

Trevenant @ Leftovers
Ability: Natural Cure
EVs: 252 HP / 180 Def / 76 Spd
Impish Nature
- Horn Leech
- Earthquake
- Will-O-Wisp
- Rest

Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 244 HP / 40 Def / 224 Spd
Impish Nature
- Substitute
- Toxic
- Protect
- Earthquake

Magnezone @ Choice Specs
Ability: Magnet Pull
EVs: 172 HP / 252 SAtk / 84 Spd
Modest Nature
- Volt Switch
- Hidden Power [Fire]
- Flash Cannon
- Thunderbolt

Cloyster @ White Herb
Ability: Skill Link
EVs: 252 Atk / 6 HP / 252 Spd
Adamant Nature
- Icicle Spear
- Ice Shard
- Rock Blast
- Shell Smash