Custom Robo/YMMV

Anticlimax Boss: Twice in Custom Robo Arena: Hadron in the sense that he is just a sooped up normal Robo, in contrast to the earlier fight with Amun (a mere gatekeeper) who is a Hover Tank with insane defense (The boss is difficult, but doesn't really differ from a standard fight). A completely intentional example is Eddy at the Robocup, as you have been shown to be superior to him alreddy in the events leading up to his fight, and is just a standard fight, no illegal parts or anything.
Game Breaker: Equip the Ray Sky with the Acrobat Pod and Plus One Legs. Use a gun with strong homing and appropriate range for the stage (Thunderbolt, Glider, Homing Star) and a bomb with appropriately large spread or blast (Burrow D, Submarine D, Volcano). Now go to the Dungeon Arena or any level of the Arcade mode and see if anyone can touch you.^[1]
- Teleport Spam + close range weapon + Teleport Spam away after they enter Mercy Invincibility is pretty damn hard for most AI foes to counter in-game.
- Many of the illegal parts can edge on this. Wyrm and Crystal strike stand out. Wyrm is basically a quadruple dragon gun that almost guarantees a downed opponent and the crystal strike renders an opponent completely helpless once it hits. The crystal strike knocks opponents skywards and has no firing lag, allowing you to hit the enemy almost endlessly. If it weren't for the point penalties in the second story mode, the game would be utterly snapped in two.
- The sniper rifle in Custom Robo: Battle Revolution fires very fast projectiles with a huge amount of stopping power, but leave you immobile for a short while after you fire it. The solution? Put it on a Lightning Sky robo and air dash. Now you can move and fire at the same time.
That One Boss: Custom Robo Arena has the two Jamesons, Kindjal and Data Hadron.
- The Gamecube version has Rahu.

↑ Use the Ray Sky's charge to launch it into the air, use the Acrobat Pod twice in succession to launch it even higher, and then, using the two air dashes to stay up there, pepper the arena with bombs and gunfire. At the end of the second dash, use the Acrobat again to down yourself, drastically reducing damage and activating Mercy Invincibility. Wash, rinse, repeat.

[1] Use the Ray Sky's charge to launch it into the air, use the Acrobat Pod twice in succession to launch it even higher, and then, using the two air dashes to stay up there, pepper the arena with bombs and gunfire. At the end of the second dash, use the Acrobat again to down yourself, drastically reducing damage and activating Mercy Invincibility. Wash, rinse, repeat.

[1]