Twice in Arena: Hadron in the sense that he is just a souped up normal Robo, in contrast to the earlier fight with Jameson, a mere gatekeeper, who is a Hover Tank with insane defense and a gargantuan spread on its gun. By comparison, Hadron is difficult, being a well-statted Lightning Sky robo with potent weapons. but fighting him isn't all that different from a standard fight. A completely intentional example is Eddy at the Robocup, as you have been shown to be superior to him already in the events leading up to his fight, and is just a standard fight, no illegal parts or anything.
The fight with Oboro in Battle Revolution in the Z syndicate's hideout where Sergei turns on Oboro and joins you in a 3v1 battle. It's built up as a climatic confrontation, but it's so stacked in your favor that the only way you might lose is if you're actively trying to.
Breather Level: The 2v1 tourneys in Battle Revolution's post game where you and an NPC ally team up to stomp an opponent. The game's narrative even frames these battles as a break from the harder tourneys that you've been competing in.
Difficulty Spike: When it is revealed that the world you know is a lie and the "real" world was destroyed generations ago the difficulty takes a noticeable increase from the rest of the game.
Even Better Sequel: While original Custom Robo was received well, V2 got even more contents like two different campaigns and new parts and robots, so it's considered as a superior sequel by most fans in Japan.
Fridge Brilliance: A question asked in Battle Revolution about the shape of the Earth is intended as a sudden surprise when everyone says all education points towards the world being flat. Your character can (and probably will) claim to the contrary and get scolded for it, making one wonder why it was even an option besides for comedy's sake. Except the Hero's father was in on the know about the Awful Truth of what happened to the world, and likely correctly taught the Hero that the world was originally round; it's not like you or the Hero had any context to say otherwise up to that point.
Fridge Horror: Various hints and implications, plus a Continuity Nod involving the Z-Syndicate and memory erasure in Arena, place Battle Revolution at the end of the series timeline. Which itself implies that unless the recurring cast members throughout the franchise before this game lived and passed on normally, everyone you came to know and even prior player characters were all murdered by Rahu. The only saving grace is the series keeps it completely ambiguous about when the end of the world happened.
Funny Moments: The overly-long "I'm not going." gag where, if you refuse about 20 times to go save the world, Harry and Marcia go on without you, promptly die, and chew you out for letting it all happen.
Game-Breaker: Since you can build the robot from many available parts, there have been overpowered parts or combinations.
Teleport Spam + close range weapon + Teleport Spam away after they enter Mercy Invincibility is pretty damn hard for most AI foes. This tactic was so deadly in V2 so Strike Vanishers are nerfed considerably the installments after Battle Revolution, but this tactic remains strong.
In V2, the Needle gun is infamous because projectiles from it launch the foe on the air and easily racks up high-damage combos by just keep firing. There is also an illegal version that performs even better.
Many of the illegal parts in Battle Revolution can edge on this, and 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.
Rahu III from Battle Revolution is extremely overpowered, even for an illegal robo. When fighting it in story mode you fight it 3v1 so it's a bit more of a fair fight, if not just barely, but once you unlock the parts for yourself there is absolutely nothing in the game that can stand up to you except maybe another Rahu III.
The sniper rifle in 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.
In Arena, 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.
The Jameson with the Sling Gun can grind up and spit out virtually any opponent in Grand Battle mode.
Heartwarming Moments: In the Gamecube version you'd think you'd have to face Rahu all by yourself again, given the game's tendency to enforce one on one battles even when it make sense for the heroes to gang up on their foes. But nope! When it comes time to literally dive into the fray, your friends mean it when they say "Everyone dive!". Harry and Marcia won't let you face Rahu alone.
Hilarious in Hindsight: Whenever you fight Dendai Don, the owner of the Chinese restaurant in Battle Revolution, the holosseum used is one that resembles a ramen bowl. A later Nintendo fighting game, ARMS, has Min Min, the mascot for a ramen shop in China, and her home stage, the Ramen Bowl.
Nightmare Fuel: Rahu was an unstoppable world-killer that destroyed everything except the town you live in, and only because it tried controlling a Robo and got stuck, giving the people a physical target. When it shows up, Battle Revolution undertakes a massive Genre Shift from fighting kooky anime syndicate villains, to a borderline Cosmic Horror Story as it outright murders people and intends on finishing its job. It's not helped by the fact that it looks like an exoskeleton with flesh-like wiring that looks more like veins and organic extensions, or that it and the destroyed world past the dome are often accompanied by a bone-chilling theme or straight up dissonant ambience.
There's also the time where, if you lose to a particular fight against it, Rahu promptly guns you down in cold blood, albeit mostly by shooting the camera and not showing the actual death before the reset.
The landing system revision in GX and Battle Revolution. In every other game, the player can at least partially manipulate their landing speeds and angles to try to optimize how they emerge from the cube, but these two opted to instead give every landing a totally random number. While you can mash through it, it can easily start a match off on a bad foot if your opponent(s) got a short timer while you have to deal with a long one.
Arena added a new mechanic in that your Robo can get dirty, both from general use and also certain conditions like plunging into lava on a certain Holosseum or using Soulboost. When your Robo got dirty, their stats were reduced, forcing the player to have to use the touchscreen and a cloth item to clean them. This is also a game that introduced actual items and currency for shops outside of battles instead of focusing entirely on Robo Parts, so naturally you either use the base Cloth to save money or tediously maintain a supply of Super Cloths to continuously make sure your robo isn't dirty as you wipe them down. Part. By. Part. Or just pay a cleaning robot to do it for you, but now that bites into your money supply for actual Parts.
The Rahu III fight is a 3-against-1 fight, and even then you still have your back to the wall, thanks to Rahu's absolutely insane statsnote It hits like a truck, is built like a damn wall, has an absurd attack range, has no knockdown recovery period and, despite the fact that it's massive enough that you'd be forgiven for mistaking it for a Metal Grappler, it's an illegal Lightning Sky model, meaning it can freely use its absurdly powerful armaments to bombard you while flying circles around your head., its holosseum being on the small side and lacking walls, and the fact that it knows that We Cannot Go On Without You. If it weren't for the fact that you have Harry and Marcia helping you out in this fight, Rahu wouldn't have much trouble mowing you down in seconds, and they don't fare much better if it's given the opportunity to go on the aggressive. This actually makes the list entirely because by comparison to the rest of the game's standards, even in the much more difficult post-game, Rahu III is just that damn hard.
That One Level: The handicap tourneys in Battle Revolution during the post game where you must fight a series of 1v2 battles. Having to take on two fully kitted out Robos on your own is one of the game's biggest challenges. Thankfully only two tourneys have this gimmick, but the second will have you playing at least two matches where two of your opponents are using illegal parts.
Visual Effects of Awesome: While the overworld graphics are usually nothing to write home about, the battle graphics were rather good with each generation of hardware, and with mostly-consistent framerates at that. Even 4-player chaos doesn't budge the Gamecube much.
gag where, if you refuse about 20 times to go save the world, Harry and Marcia go on without you, promptly die, and chew you out for letting it all happen.