Pikmin 4 has a smaller roster of major bosses than the series usually does, but each one tests a different part of your setup. The toughest fights are not about raw damage - they are about bringing the right type to the right phase.
On the smaller end of the scale sit Porquillion, Burrowing Snagret, Crusted Rumpup, Hermit Crawmad, Bug-Eyed Crawmad and Baldy Long Legs. The "tough encounters" category - the mid-tier enemies that are effectively mini-bosses - includes Tusked, Puffy, Blizzarding and Titan Blowhog, Empress Bulblax, Masterhop, the Arctic Horned Cannon Beetle, Mammoth Snootwhacker, Toxstool, and the Dwarf, Bulborb and Jumbo Bulborb family.
If you are carrying Ice Pikmin, the game gives you a whole category of immunity. Blizzarding Blowhog, Chillyhop, Freezecake, Dwarf Frosty Bulborb, Frosty Bulborb, Iceblown Dweevil, Icy Blowhog, Snowfake Fluttertail and Snowy Blowhog are all frozen-adjacent, and the Ancient Sirehound's second phase uses ice directly. Ice Geysers and Ice Vents, Ice Walls and Ice Blocks are the matching environmental hazards. Ice Pikmin and Glow Pikmin are immune to the cold; the Thermal Defense gear protects your captain and Oatchi from it.
There is no enemy or creature called an Ice Oni in Pikmin 4 - no such name appears in any official source or community data. If you have seen the term, it is almost certainly a confusion with the Ice Onion or with Ice Pikmin. This wiki does not list it as a creature.