Mega Ampharos is really slow, so a SpA boosting nature is much better than Timid. If you're set on using it as a sweeper, Trick Room is your best bet. Speed boosts aren't very effective unless you can get them consistently (like with Speed Boost) or the Pokémon's counters have been eliminated. Ampharos can't learn Trick Room, so you'll need a teammate to set it up. Bronzong is a great teammate since it can set up Trick Room, is immune to Ground-type attacks (with Levitate), and resists Ice- and Fairy-type attacks, all of which Ampharos hates. Other good teammates are bulky Water-types like Quagsire and Milotic since they can handle Ground- and Steel-types and don't fear Ice-types too much. You could use something like
Ampharos @ Ampharosite
Ability: Static --> Mold Breaker
EVs: 252 HP / 4 Def / 252 SpA
Quiet Nature (+SpA, -Spe)
IVs: 0 Spe
- Thunderbolt
- Dragon Pulse
- Focus Blast
- Power Gem / Volt Switch
Having 0 Speed IVs isn't required, but it is helpful if you can somehow get it. Use Power Gem if you want Ampharos to be a late-game sweeper after its checks are taken out and use Volt Switch if you want it to send it in earlier. If you're on the fence, Volt Switch is better overall.