Shadow Ampharos

Shadow Ampharos is a large upgrade over its base forme, but it has an issue; in-and-of itself, it's not a great attacker, and it can't Mega Evolve to turn into a good Electric Type Mega Evolution, meaning it leaves obscurity to attain mediocrity, but also sacrifices its real potential in the process. Honestly, purifying a decent Shadow Ampharos isn't a terrible option.

Best PvE Offensive Moveset

• Volt Switch outperforms Charge Beam on paper, but the latter is easier to dodge with. 
• Zap Cannon is the charge move of choice, doing far more damage for the same amount of energy as Thunder.
• Thunder Punch deals less damage per hit, but as a 3-bar move is more consistent and is a good option for a 2nd charge move. 
• Focus Blast, Dragon Pulse, Brutal Swing, Trailblaze, and Power Gem have little to no use on an offensive Ampharos.

Best PvE Defensive Moveset

• Shadow Ampharos is decent on defense thanks to Volt Switch’s huge damage; Charge Beam doesn’t hold a candle to it.
• Trailblaze has great defensive performance for Shadow Ampharos, especially thanks to its ability to counter Ground Types.
• Brutal Swing is a 3-bar move with an early damage window and good coverage, making it also an excellent defensive option
• Dragon Pulse* is another great defensive charge move. 
• Thunder Punch is generally a great option for defense due to its frequent use as a 3-bar move. However, its flexibility also leaves Shadow Ampharos extremely vulnerable to Ground Types, so it's generally not a good choice.
• Focus Blast has solid type coverage, but Shadow Ampharos probably won't get a chance to use it..
• Thunder hits fast and is hard to dodge, whereas Zap Cannon hits harder but is easy to dodge.
• Power Gem offers some coverage and can perform similarly to Dragon Pulse. 

Volt Switch is the superior fast move in both damage output and energy gains, though it with an animation duration of 2 seconds (4 turns), it can be cumbersome to use. 

Brutal Swing costs the least energy and provides coverage against Grass-, Ground-, Ghost-, and Psychic-type Pokemon. Thunder Punch is another cheap option that increases the consistency of Ampharos's Electric-type damage. Trailblaze can be used to add further pressure against Ground-type Pokemon, being particularly effective against Water/Ground-type Pokemon.

Focus Blast costs the most energy, but also threatens massive damage if unshielded. Dragon Pulse is a mediocre move that can give some coverage against opposing Dragon-type Pokemon. Power Gem adds some coverage, but it is often redundant with Ampharos's Electric STAB. Zap Cannon is largely used for highlight reels due to its nuclear damage, but it is often inconsistent for serious use. Thunder is an awkward middleground between Thunder Punch and Zap Cannon and sees no play.