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.
That Oneboss: Valgas in the first game is brutal. As with Kraken, his first and foremost goal is getting the Power Stones. Sure he won't use items often but he doesn't even NEED them, they're there to give you a chance. he hits hard, has massive health and most infuriating of all, his blows pack so much punch that more often than not every power stone knocked out of you will fly over to him, and if you have two power stones, his grab knocks them BOTH out. And unlike Kraken, his projectile attacks have homing properties and he will use super attacks that really hurt if you get hit by them.
the Pharaoh Walker in the sequel is infuriating for one major reason, you have to hit it in the head which is for the most part, impossible until you break two of its legs to knock it down. Except the people piloting are still up to interrupt your attacks with pesky projectiles and they respawn, more than likely ensuring it gets back up with only a short bit of life lost. Worst is its powerful beam attack which is the strongest attack it has and nigh impossible to dodge unless you get under it(just don't hide under the head. It can grab you with its tongue). Of course if you can grab its underside and kill the tail driver, you can use its own attack against it for massive damage.
Also from the sequel is final boss Dr.Erode. The fight takes two parts, fighting off strange tofu looking dolls who wield various weapons from the game and THEN the fight against him immediately after, so if you get beaten up by those guys then you're going to have to fight Erode at a disadvantage. Though given that they drop money and picking up their items adds it to your total pile available for fusion the tofu men have a redeeming quality, if it wasn't for the fact that there's a lot of them so if they all start moving to attack it'll hurt. As for the Doctor himself, he's just plain weird so it's hard to tell exactly what he's doing. He could spit out homing flies, he could grab you, he could grab some tea and barf it everywhere, his attacks either look the same or have little to show he's doing it, and like the Pharaoh walker, you have to hit his head(at least your characters aim for it this time). The only saving graces are that cake will always spawn if you break certain items and that he doesn't take Power Fusion attacks very well.