Mareep is available only as a Horde Encounter on Route 12, at a 35% chance. Horde encounters may be triggered by using Honey or Sweet Scent.
Mega Ampharos' Ability is also an apt description of the line as a whole, being slow but bulky in contrast to the "Fragile Speedster" stats of typical Electric types. But with Speed being it's only major flaw it's a role it fills well. Plus for those whose rules allow for Mega Evolution will have access to one of the highest Special Attack stats of all Pokémon.
Rival (Tower of Mastery): If you still have a Flaaffy, the only thing it can fight is Frogadier- Thundershock simply doesn't cut it against the others. Ampharos, on the other hand, can Thunder Punch down anything not named Quilladin.
Gym #3 - Korrina (Shalour City, Fighting-type): Flaaffy need not apply, but Ampharos can simply spam Thunder Punch against everything, possibly without even needing to heal. Isn't bulk wonderful? ...Er, just don't paralyze the Machoke; the chance of cancelling attacks is not worth triggering Guts.
Rival (Coumarine City): Their team is exactly the same as before. You now will definitely have an Ampharos, possibly with its Mega Stone. This will be easy. (Though you should probably wait until Meowstic's down to Mega Evolve, because of Disarming Voice.) Quilladin still takes a while to beat, though you can temporarily teach Ampharos Power-Up Punch to speed the process.
Gym #4 - Ramos (Coumarine City, Grass-type): Ampharos, Mega'd or not, will tear down Jumpluff with Thunder Punch and Weepinbell with Return. Gogoat shouldn't be fought unless you've already gotten a boost from Power-Up Punch; it's too much an even fight without it.
Aliana (Kalos Power Plant): Mightyena's hardly intimidating (heh), not with Ampharos's bulk. A couple of Thunder Punches or Power Gems will mop it right up. The worst you have to worry about is Swagger hax.
Gym #5 - Clemont (Lumiose City, Electric-type): Power Gem for Emolga and Power Up Punch for Magneton and Heliolisk. Being able to resist many of his attacks is a major boon for Ampharos. Mega Ampharos is just twisting the knife.
Rival (Route 14): Meowstic's too durable for Ampharos if it gets its Light Screen up; Mega Ampharos can handle it though. Absol's frail as ever; let the Thunderbolts fly! Greninja naturally has no chance and Delphox fares little better but Chesnaught's best left to a different Pokémon as Ampharos simply doesn't have very effective moves against it.
Gym #6 - Valerie (Laverre City, Fairy-type): Mawile's poor Special Defense make it a Thunderbolt magnet and with it lacking Fairy STAB you can safely go Mega against it. Then again, that's all the use Ampharos will see; Mr. Mime and Sylveon hit too hard even on a non-Mega'd Ampharos.
Celosia and Bryony (Poké Ball Factory, tag battle with rival): Take out Manectric with Power Gem first (because of Lightning Rod) and then Thunderbolt the Liepard into oblivion. Easy.
Mable (Frost Cavern): Mega'd or not, Ampharos can crush Houndoom with Power Gem.
Rival (Anistar City): Thunderbolt fries Meowstic, Absol, Flareon, Greninja and Vaporeon; it'll take care of Delphox as well, but only a Mega Ampharos will have the durability to survive in the meantime. Dragon Pulse or Power Gem takes out Jolteon who can do very little in response. Mega Ampharos (and the Mega evolution is necessary) can triumph over Chesnaught with Dragon Pulse, if you have it, but shouldn't participate if it doesn't.
Gym #7 - Olympia (Anistar City, Psychic-type): Ampharos fries Sigilyph with Thunderbolt even if it's hiding behind a Light Screen. Slowking should only be fought if Light Screen's down, or it'll be able to tank long enough to Calm Mind its way to dangerous levels. Thunderbolt will take care of a screenless Meowstic, but if Light Screen's still up only Mega Ampharos can manage it.
Lysandre battle #1 (Lysandre Labs): Mienfoo and Murkrow die to Thunderbolt like they were nothing. Pyroar can only be safely taken on with a Mega Ampharos (with Power Gem) if the Ampharos is Male, but a female can handle him even non-mega'd. Thunderbolt + Gyarados = enough fried fish for a dozen Fridays, but be cautious; Ampharos can survive a regular Earthquake but not a critical one.
Aliana (Lysandre Labs): Mightyena's as ineffectual as before. Druddigon, if you can outspeed it, can be nailed with Dragon Pulses, though for safety's sake you might want to use Mightyena to set up a Cotton Guard or two first (assuming it lets you.) And don't Mega Evolve; the added bulk's not worth the extra Dragon weakness.
Celosia (Lysandre Labs): Dragon Pulse for Manectric, Thunderbolt for Drapion. The biggest worry is Drapion getting lucky with Acupressure; then again that applies regardless of what 'mon you throw at it.
Bryony (Lysandre Labs): Who has no thumbs and gets OHKO's by Thunderbolt? Liepard! Bisharp doesn't fair much better and even crit Night Slash won't kill Ampharos.
Mable (Lysandre Labs): Power Gems all around, though a non-Mega'd Ampharos will probably have to heal in-between.
Xerosic (Lysandre Labs): Crobat fries in the face of Thunderbolt, to the surprise of no one. For that matter, Malamar can be handled the same way (or if you've overleveled a bit, with Signal Beam.)
Lysandre battle #2 (Team Flare Secret HQ): Same business as before, other than Pyroar has switched abilities and can now be safely handled by even a non-Mega'd Ampharos.
Xerneas (Team Flare Secret HQ, X only): Ah, no. Xerneas is simply too heavy a hitter for Ampharos, and Mega Evolving is off limits for obvious reasons.
Yveltal (Team Flare Secret HQ, Y only): Thunderbolt, naturally, though only Mega Ampharos is durable enough to pull it off.
Lysandre battle #3 (Team Flare Secret HQ): Same as the previous encounter, other than you definitely want to avoid the now-Mega Gyarados, now that it can no longer be one-shotted.
Professor Sycamore (Couriway Town): A Mega Ampharos with Dragon Pulse should be able to handle Venusaur. (Regular ones will find Venusaur's bulk hard to break through.) Either can shut down Charizard and Blastoise with Thunderbolt, though obviously a Mega Ampharos is preferred for the extra layer of resistance.
Shauna (Route 19): Delcatty, poorly stat-ed as it is, goes down quickly to a Thunderbolt or two,preferably from a regular Ampharos due to its Play Rough. Avoid Goodra - it has Earthquake and Special Defense up the wazoo. As for the starters, Thunderbolt Greninja, avoid Chesnaught and chuck Power Gems at Delphox (though in this last case regular Ampharos should retreat if its Special Defense is lowered.)
Tierno (Route 19): Power Gem crushes Talonflame, who can't do much damage anyways. Dragon Pulse for Roserade, but only a Mega Ampharos need apply. Crawdaunt is a sitting duck crustacean for Thunderbolt.
Trevor (Route 19): Raichu can do very little - Dragon Pulse it to death. Aerodactyl should be felled in a single Thunderbolt and apart from Supersonic annoyance is pretty non-threatening. Don't battle Florges as she can tank quite a few Thunderbolts and pack quite a punch with Moonblast.
Gym #8 - Wulfric (Snowbelle City, Ice-type): As long as Ampharos (non-Mega'd, natch) is faster, it should be able to take out Abomasnow with Power Gems. Success against Cryogonal largely depends on how much Confuse Ray screws you over; holding a Persim Berry may be a good idea. Avalugg's low Special Defense means it'll be shattered by Power Gem.
Rival (Victory Road): This plays out mostly the same as the previous rival fight. The main differences are that they have an Altaria (which a non-Mega Ampharos can take out with Dragon Pulse) and Greninja now has Ice Beam, meaning Mega Ampharos should exercise caution around it.
Elite Four Wikstrom (Pokémon League, Ironworks Chamber, Steel-type): Klefki can be fried with Thunderbolts (from a non-Mega'd Ampharos, since Dazzling Gleam is a thing), though since it will most likely Torment you you'll have to keep alternating moves. Avoid Probopass because of Earth Power. Thunderbolts for the Scizor and Aegislash, though only if Ampharos's health is high as the former's X-Scissor and the latter's Shadow Claw will take off a significant amount.
Elite Four Malva (Pokémon League, Blazing Chamber, Fire-type): Power Gem will take care of Pyroar, though with Rivalry in play female Ampharos WILL have to Mega Evolve to survive. (You'll also want to switch out afterward to clear up Noble Roar drops.) Even regular Ampharos can survive a one-Curse-boosted Earthquake from Torkoal and two Power Gems will kill it so you should be okay. Chandelure should only be fought by a Mega Ampharos and even then Confuse Ray and Confide can easily trip you up; proceed with caution. Talonflame utterly crumples in the face of Power Gem.
Elite Four Drasna (Pokémon League, Dragonmark Chamber, Dragon-type): For obvious reasons Ampharos is gonna want to stay unmega'd for this fight. Despite that, Dragon Pulse proves an effective weapon against every one of her Pokémon; however Noivern's better off avoided as it's a very close matchup that could turn disastrous with a bad dice roll.
Elite Four Siebold (Pokémon League, Flood Chamber, Water-type): This, right here, is Ampharos's playground. Thunderbolt will plow through everything here. The main worry here is Gyarados's Earthquake, but Ampharos can survive one as long as it doesn't crit.
Champion Diantha (Pokémon League, Radiant Chamber): Hawlucha dies to Thunderbolt, period. Dragon Pulse for Tyrantrum, but only as Mega Ampharos as a regular Ampharos is not guaranteed to OHKO it and you can't survive two Earthquakes. Aurorus is Power Gemable, though it'll take patience and most likely healing if it puts up a Light Screen. Gourgeist's Trick-or-Treat/Phantom Force combo will probably prove too much for Ampharos. Goodra's only manageable with healing, but if you're willing to spend the Full Restores, Dragon Pulse away. Mega Gardevoir's much too heavy a hitter with a truckload of Special Defense to boot.
Post-Game: Ampharos continues to shine in the post-game. Just keep its Mega Evolved form away from Essentia's Fairy types and it'll do fine.
At the level you catch it at (13) it'll have Growl, Thunder Wave, Thunder Shock and Cotton Spore, though Thunder Wave's probably the only one to hold onto. At level 15 it learns Charge (and more importantly evolves) which might be handy until it's able to go more on the offensive. At level 20 it learns Take Down; not worth it, especially when you already have access to Return. At level 25 it learns Electro Ball - terrible given the line's slowness but still a step up from Thundershock. At level 29 it learns Confuse Ray which is in the same boat as Charge. At level 30 it evolves again and learns Thunder Punch; physical, but it'll have to do for now. At level 35 it learns Power Gem; hold onto it forever as there are a surprisingly large amount of bosses with Fire types. At level 40 it learns Discharge, though depending on how much you've abused EXP Share you'll probably already have Thunderbolt. At level 46 it learns Cotton Guard which is superb especially for -Def natures. At level 51 it learns Signal Beam, but honestly it doesn't see much use. At level 57 it learns Light Screen good for a more support-oriented Ampharos, or one with weak Special Defense. At level 62 it learns Thunder but apart from Rain Dance sets it's not worth it. Finally at level 65 it learns Dragon Pulse...except it can (and should) have already relearned it long ago.
Now then, TMs! Thunderbolt is pretty much a given. Power-Up Punch is useful in the short run to boost Thunder Punch. Rain Dance may be used in combination with Thunder (which is also a TM) if desired. Lastly, Focus Blast for those willing to risk its poor accuracy and Hyper Beam for a powerful finishing move, assuming you're on Switch mode.