TM91 Flash Cannon can be found on a cliff on a specific Mirage Island in the northeast region of Hoenn. But you can only explore Mirage Islands after you unlock the ability to soar with Mega Latias/Latios by using the Eon Flute.
As for the move itself:
Flash Cannon is a damage-dealing Steel-type move that has a small chance to lower the target’s Sp. Def. It’s by far the most reliable Special Steel-type move in the game. Additionally, it can potentially make taking down specially defensive walls easier.
You’ll acquire the Eon Flute from Steven shortly after you battle Kyogre or Groudon in the Cave of Origin. This Key Item will let you call on Latias/Latios (depending on your game version) to Mega Evolve and soar through Hoenn’s skies.
Soaring lets you quickly travel to previously visited locations and also grants you access to Mirage Spots.
Mirage Spots are rare locations that can randomly be found throughout the Hoenn region. They provide various benefits, such as a way to catch rare or legendary Pokémon, acquire valuable items, and collect unique TMs, including TM91 Flash Cannon.
A random Mirage Spot appears each day around Hoenn. You’ll see them marked by a red sparkle (pictured above).
The Mirage Spots rotate daily, so if your current save file does not have the Mirage Island for TM91 then you can always try again tomorrow (or speed up time in-game).
TM91 Flash Cannon Location (Step-by-Step)
When you’re good to go, check and see if the exact Mirage Island marked in the image below is available.
When it is, use the Eon Flute to travel there.
If it’s not available then you can adjust your 3DS or emulator’s time until it appears.
Step 1: When you arrive at this Mirage Island, head southeast towards the tall grass.
Step 2: Proceed all the way eastward, passing by the ruins, and then climb the nearby ladder.
Step 3: Once you’ve climbed the ladder, go north into the ruins and take a left towards the small cliff. You’ll find the Flash Cannon TM on the ground near the edge.
TM91 Flash Cannon Details + Uses
|Flash Cannon Move Details|
|PP||10 (max. 16)|
TM91 Flash Cannon is a Special Steel-type move with a 10% chance of reducing the target’s Special Defense by one stage.
Stats can be lowered up to six times in battle. Here’s how each successful proc of Flash Cannon’s effect can impact the stat value at each stage:
With a base power of 80 and a perfect accuracy, Flash Cannon is certainly the most reliable Special STAB move for Steel-type Special attackers.
However, this is mostly because Flash Cannon is one of only three Special Steel-type moves in the game, alongside Mirror Shot and Doom Desire.
Regardless, Flash Cannon remains a great choice for the Pokémon that can learn it.
Although Flash Cannon’s secondary effect doesn’t trigger often, the possibility of lowering an opponent’s Special Defense can help your team break through specially defensive walls more easily.
Flash Cannon is also particularly valuable due to its effectiveness against Fairy-types, which are quite prevalent in competitive battles.