Having more than 510 EVs is impossible, as seen with Tangrowth:
-Giga Drain/Leaf Storm
-Knock Off
-Earthquake
-Hidden Power (Ice/Fire)/Rock Slide
Nature: Relaxed
EVs: 252 HP/4 Def/252 SDef
Item: Assault Vest
Ability: Regenerator
or
-Giga Drain
-Knock Off
-Sleep Powder/Earthquake
-Hidden Power (Ice/Fire)
Nature: Relaxed
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Regenerator
Focus Blast is a necessity to deal with Ferrothorn and Excadrill if Mega Ampharos is to be on the offensive. Also, what purpose does your current EV spread serve? RestTalk is viable on Mega Ampharos; though it'll be walled by the Pokemon I mentioned. RestTalk Mega Amphy is an offensive pivot:
-Rest
-Sleep Talk
-Volt Switch
-Dragon Pulse
Nature: Modest
EVs: 252 HP/44 SAtk/192 SDef/20 Spe (2HKOs Hippowdon with Dragon Pulse while outspeeding it at the same time)
Item: Ampharosite
Ability: Static
or
-Volt Switch
-Dragon Pulse
-Focus Blast
-Thunderbolt/Heal Bell
Nature: Modest
EVs: 236 HP/252 SAtk/20 Spe
Item: Ampharosite
Ability: Static
Honestly, I don't like having Magcargo in an OU team. Its common weaknesses to Water, Fighting and Ground do it no favors, as those types of attacks are all over the place. Shell Smash + Recover is a big no-no; Magcargo won't have time to Recover off any damage it sustains if it Shell Smashes, plus it takes up a moveslot that would've gone to an extra coverage move. It's also too slow for Shell Smash in the first place (Torkoal may be slower than Magcargo, but it gets away with Shell Smash because it threatens the spin blockers in NU). As a result of this, Magcargo is better off using a defensive set, despite its bad defensive typing (at least it checks Talonflame):
-Stealth Rock
-Lava Plume
-Toxic
-Recover
Nature: Bold
EVs: 248 HP/8 Def/252 SDef
Item: Leftovers
Ability: Flame Body
Given how Magcargo is vulnerable to all entry hazards, Rapid Spin or Defog support is a necessity:
Starmie:
-Hydro Pump/Surf
-Thunderbolt/Psyshock
-Ice Beam
-Rapid Spin
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Leftovers/Life Orb
Ability: Natural Cure
or
-Scald
-Rapid Spin
-Psyshock/Ice Beam/Thunderbolt
-Recover
Nature: Timid
EVs: 248 HP/32 Def/4 SAtk/224 Spe
Item: Leftovers
Ability: Natural Cure
Excadrill:
-Earthquake
-Rock Slide/Iron Head
-Rapid Spin
-Swords Dance/Stealth Rock
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Air Balloon/Chople Berry
Ability: Mold Breaker
or
-Swords Dance
-Earthquake
-Rapid Spin
-Rock Slide/Substitute/Iron Head
Nature: Adamant
EVs: 176 HP/60 Atk/252 SDef/20 Spe
Item: Leftovers
Ability: Mold Breaker
Mandibuzz:
-Foul Play
-Roost
-Taunt/Whirlwind
-Defog/Toxic/Knock Off
Nature: Bold/Impish
EVs: 248 HP/216 Def/44 SDef
Item: Leftovers
Ability: Overcoat
Crobat:
-Brave Bird
-Defog
-Roost
-Taunt/Toxic
Nature: Jolly
EVs: 168 HP/136 Atk/4 SDef/200 Spe
Item: Black Sludge
Ability: Infiltrator
Scizor:
-U-turn
-Bullet Punch/Knock Off
-Defog
-Roost
Nature: Impish
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Technician
Tentacruel:
-Scald
-Sludge Bomb
-Rapid Spin
-Knock Off/Toxic
Nature: Bold
EVs: 252 HP/224 Def/32 Spe
Item: Black Sludge
Ability: Liquid Ooze