Set 1:

Eelektross @ Waterium Z  
Ability: Levitate  
EVs: 252 HP / 24 Atk / 120 Def / 112 SpD  
Adamant Nature  
- Coil  
- Aqua Tail  
- Substitute  
- Rock Tomb

What it beats:

Golem-Alola
    24+ Atk Eelektross Rock Tomb vs. 0 HP / 0 Def Golem-Alola: 44-52 (14.6 - 17.2%) -- possible 6HKO 
    24+ Atk Eelektross Hydro Vortex (175 BP) vs. 0 HP / 0 Def Golem-Alola: 254-300 (84.3 - 99.6%) -- guaranteed 2HKO 
    252+ Atk Golem-Alola Continental Crush (180 BP) vs. 252 HP / 120 Def Eelektross: 318-375 (85 - 100.2%) -- 6.3% chance to OHKO 

(Rock Tomb into Z-Stone Edge beats Eelektross however)

Stunfisk
    24+ Atk Eelektross Hydro Vortex (175 BP) vs. 252 HP / 0 Def Stunfisk: 368-434 (87.2 - 102.8%) -- 18.8% chance to OHKO 
    252+ SpA Stunfisk Surf vs. 252 HP / 112 SpD Eelektross: 83-98 (22.1 - 26.2%) -- 11.1% chance to 4HKO 
Rotom-Heat (but that's not relevant at all so I won't do calcs for it)

Set 2:

Eelektross @ Firium Z  
Ability: Levitate  
EVs: 252 HP / 120 Def / 24 SpA / 112 SpD  
Modest Nature  
IVs: 30 Atk  
- Flamethrower  
- Hidden Power [Ground]  
- Rock Tomb  
- Super Fang 

What it beats:

Rotom-Mow
       252+ SpA Rotom-Mow Bloom Doom (195 BP) vs. 252 HP / 112 SpD Eelektross: 316-373 (84.4 - 99.7%) -- guaranteed 2HKO 
       24+ SpA Eelektross Inferno Overdrive (175 BP) vs. 0 HP / 4 SpD Rotom-Mow: 278-328 (115.3 - 136%) -- guaranteed OHKO 

Golem-Alola
       0- Atk Eelektross Rock Tomb vs. 0 HP / 0 Def Golem-Alola: 35-42 (11.6 - 13.9%) -- possible 8HKO 
       24+ SpA Eelektross Hidden Power Ground vs. 0 HP / 4 SpD Golem-Alola: 288-340 (95.6 - 112.9%) -- 68.8% chance to OHKO 
       252+ Atk Golem-Alola Continental Crush (180 BP) vs. 252 HP / 120 Def Eelektross: 318-375 (85 - 100.2%) -- 6.3% chance to OHKO

Vikavolt
       24+ SpA Eelektross Inferno Overdrive (175 BP) vs. 248 HP / 8+ SpD Vikavolt: 338-398 (94.6 - 111.4%) -- 68.8% chance to OHKO 

Set 3:

Eelektross @ Fightinium Z  
Ability: Levitate  
EVs: 252 HP / 48 Atk / 48 Def / 160 SpD  
Impish Nature  
- Superpower  
- Coil  
- Rock Tomb  
- Super Fang  

What it beats:

Golem-Alola
     48 Atk Eelektross Rock Tomb vs. 0 HP / 0 Def Golem-Alola: 41-49 (13.6 - 16.2%) -- possible 7HKO 
     48 Atk Eelektross All-Out Pummeling (190 BP) vs. 0 HP / 0 Def Golem-Alola: 256-302 (85 - 100.3%) -- 6.3% chance to OHKO 
     252+ Atk Golem-Alola Continental Crush (180 BP) vs. 252 HP / 48+ Def Eelektross: 315-372 (84.2 - 99.4%) -- guaranteed 2HKO 

Heliolisk
     252+ SpA Heliolisk Breakneck Blitz (200 BP) vs. 252 HP / 160 SpD Eelektross: 316-373 (84.4 - 99.7%) -- guaranteed 2HKO 
     48 Atk Eelektross All-Out Pummeling (190 BP) vs. 0 HP / 4 Def Heliolisk: 534-630 (201.5 - 237.7%) -- guaranteed OHKO 

Why Eelektross:

It has a lot of coverage. It can hit a lot of the tier for super effective with z-moves. This makes it very versatile.
It has levitate which makes you immune to z-ground, one of the more common moves in the tier.
It has reasonable bulk, and it has no weaknesses with access to coil to boost it's bulk even further while raising it's attack.