The Pikachu Cup is gonna be so fun. I spent quite a bit of time building my team and compering notes with others, so I'll try and pass on some of that knowledge ;) Changes will be in bold.
Lanturn@ Sitrus Berry
Bold | Volt Absorb
244 HP / 204 Def / 4 SpA / 52 SpD / 4 Spe
- Soak
- Scald
- Icy Wind
- Protect
Lanturn's good. All I did was optimize your EV spread, as you were wasting a few stat points in Defence. Just a warning that the Soak strategy was a bit overhyped and a lot of players are aware of that now. If you see an Electrode, don't bring Soak as you will be Taunted.
Rotom-Mow@ Life Orb
Modest | Levitate
132 Def / 252 SpA / 124 SpD
- Leaf Storm
- Hidden Power [Ground]
- Will-o-Wisp
- Protect
Rotom's good too. I'm curious about your EV spread and what it survives. You could probably save a lot of EVs by investing more in HP.
Emolga@ Focus Sash
Jolly | Motor Drive
252 HP / 4 SpD / 252 Spe
- Knock Off
- Tailwind
- Helping Hand
- Protect
Pikachu is awful. It is greatly inferior to both Emolga and Air Balloon Raichu as a support Pokémon. The event Pikachu also holds little value as the only thing special about it is Endeavor. That's why I suggest a different support Pokémon. Emolga + Electivire is an incredibly strong duo, with Helping Hand + Earthquake decimating everything not immmune. Tailwind offers some speed control, letting you win Ampharos mirrors, Electivire mirrors, etc. Knock Off is for disruption. Alternatively, you could use Air Balloon Raichu for fast Encore disruption, but I though Emolga would serve you better.
Electivire@ Choice Band
Adamant | Motor Drive
4 HP / 252 Atk / 252 Spe
- Earthquake
- Ice Punch
- Cross Chop
- Rock Slide
Electivire is a monster. With two types of speed control, Tailwind and Icy Wind, to me it seems that Choice Scarf is a little overkill and that a Choice Band will suit you better. Cross Chop is stronger than Brick Break and Light Screen / Reflect aren't going to be terribly common. Rock Slide is for flinches under Tailwind, as well as hitting Rotom-Heat and Emolga. Taunt is bad on Electivire, and on Choiced Pokémon in general. If you want a fast Taunter, try Electrode.
Ampharos@ Ampharosite
Modest | Plus
244 HP / 28 Def / 204 SpA / 28 SpD / 4 Spe
- Dragon Pulse
- Thunderbolt
- Hidden Power [Ground]
- Protect
Ampharos is fine, the EV spread just needed optimizing. If you ever drop the Soak strategy, abandon Thunderbolt for Agility, Light Screen, or Power Gem. It's also worth noting how Dragon Pulse will hit harder than HP Ground on everything but Rotom-Heat and Magneton/Magnezone.
Eelektross@ Assault Vest
Adamant | Levitate
252 HP / 4 Def / 212 Atk / 4 SpD / 36 Spe
- Super Fang
- Dragon Claw
- Drain Punch
- Power-Up Punch / Fire Punch
Eelektross is some sort of god in this competition. It's coverage is massive and Super Fang chops down bulky threats like Ampharos and Rotom. The EV spread should outspeed your Ampharos by one point after Mega Evolving, letting you attack with Super Fang + Dragon Pulse for a lot of KOs. Assault Vest is probably the best item for Eelektross, and the extra bulk will likely be more useful than Protect in a lot of situations. For the last slot, Power-Up Punch boosts the power of Drain Punch and Dragon Claw, while Fire Punch hammers Rotom-Mow and Galvantula.
This was a really fun rate! I hope you have fun in the Pikachu Cup, and that my advice is useful for a few battles. See you on Battle Spot!