Aegislash - Shield

• First up, Aegislash demands shields and/or switches in order to survive once it's turned into its paper-like Blade forme. Switches in particular are especially true when your opponent is packing DPS-focused Fast Moves, as Aegislash - Blade Forme is easily farmed down in that scenario due to its own middling Fast Move damage output, while on the shield-usage side it's likely dead even if it takes a resisted Charged Move. There's also a scenario where you don't have a switch available and the opponent doesn't drop a Charged Move, instead farming down Blade Forme and killing it with Fast Move damage before it can throw out another Shadow Ball. It's risky, as Aegislash could build up to another Shadow Ball before the job is done, but it can definitely happen.

• Second, Aegislash is surprisingly passive. While it's building up energy, it's exerting absolutely no pressure with its Fast Moves, and it wants to get as close to charging up 2 Shadow Balls as it can in order to make the most of its time in Blade forme. And while it's making these preparations, the opposition is taking pot-shots with Fast Moves that cut down Aegislash's HP faster than Aegislash can return in kind despite Shield forme's impressive bulk, and they're also building their own energy at the same time.

• Third, and sort of connected to second, Aegislash absolutely needs help dealing with shields! Even if you can rapid-fire two Shadow Balls, you've done absolutely no Fast Move damage, and you've strictly given your opponent a reason to use their own shields twice in a row and basically sacrificed your likely already wounded Aegislash unless you switch it out immediately after. Sure, this strategy can have value, but let's look back to the second point and it's plain to see that your opponent likely has 50-70 energy by this point, assuming they haven't already unloaded their own Charged Move. And speaking of that....
• Fourth, you really only want to take a Charged Move when you've got shields left and you're in Blade forme. If you shield while in Shield forme, you've basically sacrificed one of 3-4 times that you can pull off Aegislash's big gimmick, greatly hindering what it's capable of doing in most cases.
• Fifth, Aegislash is really, really predictable. If you see this thing take the field, you know EXACTLY what it's here to do, and you can play around it as such to ruin its day without having to worry about any sort of tactics like coverage moves that aren't Steel.
• Sixth, while it has Psycho Cut for faster energy generation... THIS THING GENERATES ENERGY SLOWER THAN ANY OTHER POKEMON WITH THE MOVE! It has a built-in UNADVERTISED energy generation nerf that drives it into the ground and ensures that it can't even get to its Charged Moves at a decent rate!

In short, Aegislash... was very much a fumble. Whoever designed this thing severely botched its implementation (and that's not just referring to the fact that it was glitched on release to take greatly reduced damage in Shield forme), and didn't even explain its unique gimmick in-game. It could greatly improve in the future if given a better moveset (worth noting; it can learn Sacred Sword in the main series and Ghost/Fighting have perfect coverage together outside of Hisuian Zoroark!) and the ability to actually deal damage and gain a normal amount of energy with its Fast Moves in Shield forme (Shadow Claw would be nice!), but for the time being it's just a broken gimmick that has a high skill-cap for mediocre payoff in many cases.