Diamond Long Legs

The Diamond Long Legs is a miniboss encountered in Pikmin 4 (Wii U). It is the rarest arachnorb in the game, only appearing once in Silver Lake. It also appears in two caves: Chaotic Cove and Spider's Web. It is located in a snowy arena to the east of the landing site, where there are gems arranged into the shape of a web. Rock Pikmin are recommended to fight the creature with as it has a covering made of diamond. When defeated, it drops the Snow Parka upgrade.

The Diamond Long Legs cannot be fought without using Rock Pikmin. Rock Pikmin are also immune to being crushed and stabbed, only making them more useful in fighting the creature. To defeat it, simply thrown Rock Pikmin at its protected core until its crystal coating shatters, and hit the main body to inflict damage to the boss. Whenever its armor is broken, shards of it will come crashing down to the ground, impaling any Pikmin unfortunate enough to be under them as they land. The coating will eventually reform and must be broken again.

BallsAngus' notes

 * "This species of arachnorb uses gems or hard minerals as a defense mechanism. It uses the minerals as armor to protect itself from enemies. When its armor is removed, it is vulnerable to attacks. Recent studies have also shown it has the ability to regrow its diamond covering, but it takes a lot of time, which is why it must be defeated as soon as possible when its squishy core is exposed.

In Pikmin 10
In Pikmin 10, the Diamond Long Legs is a bit more common. It moves incredibly slowly. Much like in Pikmin 4 (Wii U), hitting its crystal armor until it breaks will reveal its main body, which is vulnerable to attack. However, the Diamond Long Legs will move much faster when its armor is broken. Breaking the bits of crystal on its joints will deal a fair bit of damage to the boss. When defeated, the Diamond Long Legs will roar, stiffen up, drop whatever it was holding, and then completely shatter. Any joint coverings that were not destroyed will also shatter. The crystal shards will knock down Pikmin and leaders they land on.