These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Cosmic Break
Alt-itis - A recent update allowed players to receive a single item that can generate a random Humanoid. At it turns out, many MANY powerful humanoids are possabillites (Including the supposedly never to be released again Type Zeroes), cue a large majority of the player-base making alternate accounts to get a chance at a powerful Humanoid.
Demonfox Haku fights you as a extremely fast baby Haku.
In beta, for the renowned Action Bomb C.S-Chan's fight, she usually summons powerful robots to fight for her. On zero-star difficulty, she summons two Squidols, which have tentacles for their arm and leg slots. Then they put their parts to good use, which prompts the Action Bomb Loli Boss to activate her Self-Destruct Mechanism on the Squidols.
Fractulus and Berzelius are both tiny in their 0* boss fights as well, but it's quite different from Baby Haku.
Demonic Spiders - Players sporting low-liability bots with ridiculous weapons such as Blast Bazookas and Gatling Bazookas. Can front load hundreds of damage when used by smart players, and when you kill them they're barely worth anything. And they have more where that came from.
Done quite literally with Tarantulic, a spider bot with Web Gun, which inflicts the very annoying slow status effect. From Bad to Worse it goes if you put Winberrl arms on him, leading to stun, slow, AND being pulled in.
Vanguard Fencer, a RT only bot, does incredible damage even fresh out of the Garapon, and tends to stun anything he gets in range of, leading to a swift death. Fortunately, he has absolutely no defense against ranged attacks.
Goddamned Bats - They live on Arcantus. Also come in the form of Pterodactyls with breaker rifles in their heads.
Demonfox Haku is just made of this trope. All of his attacks either shave off about half your HP, break off some of your parts, inflict a debuff or just plain kill you.
Action Bomb C.S-chan who blows herself up and instantly kills everyone in range at low health, and you're likely to be a low health before killing her, because her earlier attacks are hard to avoid.
A non-boss example, instead one you can perform yourself if you get a Seraph Crimrose FC (and immediately earn the ire of every player on the enemy team if you do): she comes with a subweapon that locks onto six separate targets and fires a blue homing blast at each of them, sucking them in and doing well over 100 damage to each. The best part is that it's not even an Rt bot, it's obtained through pure UC (and a lot of luck in fishing a Seraph FC Promo from the UC Gara DX). Falls under Necessary Evil in the event of you not having anything else that can compete in the metagame, which is likely if you don't shell out cash for Rt.
That One Level - Temple Site. The map itself will kill more players than anything else. Don't be surprised if the participant number goes down when it comes up in the cue.
In the betas, Ivis was this. Her quick-striking, well-tracking, hard-hitting rapier "Sacrifice" was bar-none the best melee attack in the game, and she was also the progenitor of the Crimson Veil ability which made it nigh-impossible to stun her. Worse yet, unlike the Revenge tune version later introduced in official service, her Crimson Veil increased her attack range.
Out of beta, while still generally regarded as the Queen of Melee, Ivis' Tier-Induced Scrappy status has been significantly reduced... mainly because Aquila Girl is much, much worse. Simply locking onto an enemy with her Homing Blaster guaranteed around 40 damage with each shot, and she introduced the Full Boost ability which lets her recharge her Boost Gauge and maxes out her FLY stat. With Propeller Bits, she can stay in the air and rain green death for a very long time.
Rounding out the basic Rock-Paper-ScissorsTier-Induced Scrappy trio as an ART is Lily Rain EVE due to her shield ability, insane damage output, and good mobility (high FLY stat, as well as the Float Dash cartridge). She's not as bad as the other two though, as being earthbound without a melee alternative (usually) she has to make every shot count, which is a difficult task while having to avoid the swarms of LNDs usually scurrying about, especially Ivis. Because of this notable amount of effort required to accomplish this nowadays, people who can play Lily Rain EVE well are usually applauded rather than scorned.
Then came the FC (Full-Color) version of Seraph Crimrose... she has the same abilities as the original (namely Seraph Wing, similar to Full Boost but it also increases her flight time by a lot and gives her the Anti-Homing buff), but with one more added feature: Seraph Nova, a subweapon that targets up to six enemies at once and launches an energy blast that sucks them in and damages them pretty badly. It was so deadly that CyberStep had to decrease the sucking and force of the attack within a week of her introduction.
Fortunately, at least Seraph Crimrose FC was - and still is - available by pure UC so players not willing to shell out money on the game can get something that can compete in the current meta.
And these previous examples say nothing of what came out before Seraph Crimrose FC: the first ZERO-type robots, Resha and Eris.
Eris was a top contender for taking away Ivis' Queen of Melee title. She wielded two large swords with deadly efficiency and, depending on the cartridge path used, could either user her wings to negate all damage or turn almost completely invisible. However, she was manageable due to all the regular anti-melee techniques still working on her.
Resha, on the other hand, was far and away the most powerful "robot" in the game for a number of reasons. She could launch a myriad of homing lasers, create very sturdy barriers for herself (and depending on the cartridge path used, her allies as well), swing her staff with considerable force and, most notably, launch a Fantastic not-quite Nuke that hurt like hell, had an absurd AOE and tracking and jammed the guns of everyone hit. She was so overpowered that pretty much the entire Cosmic Break playerbase demanded that she be nerfed, even the players who used her with wild abandon. The Japanese server even protested with in-game signs! Eventually the players got their wish, and Resha and a handful of other recently-introduced and complained about robots were toned down.
Most recently, Ivis and Crimrose got another pair of revamps that seem to be headed in this direction.
Seraph Crimrose SIGMA, in addition to now looking like an admittedly Badass mecha-musume, now has access to a pair of Beam Spears that provide a Homing Blaster and Sword Beams; she can also spin and throw the spears to simulate a single-target Seraph Nova, but obviously not as powerful. Add on her trademark Seraph Wing, and she's arguably the best AIR-melee in the game with a significant amount of ranged punishment on the side.