A Shroob Rex is a purple Shroob-like Rex found in Shroob Castle, during the events of Mario & Luigi: Partners in Time. Shroob Rexes never appear in the overworld, they only appear in battle alongside other enemies.
In battle, they use a wide variety of attacks. First, they jump, then Ground Pound, creating an energy wave. This attack can be dodged by Jumping. However, depending on where the Shroob Rex Ground Pounds, the timing changes. If the Shroob Rex ground pounds closer to Luigi, he must jump first, followed by Mario. If closer to Mario, he must jump first, followed by Luigi. If the Shroob Rex Ground Pounds at an equal distance between the two, both Mario Bros. must jump at the same time to avoid the energy wave. This attack can cause the Trip status ailment.
A Shroob Rex can also shoot boulders at the Mario Bros. These boulders can be deflected with a Hammer. When shot, the boulder may split in two, requiring both Baby Mario Bros. to swing their Hammers at the same time. Additionally, the Shroob Rex can fire one boulder that homes in on one Mario Bro. When deflected, the boulder rolls into the second Mario brother, and must be deflected again. It homes in on Mario first when it lifts its head up before spitting out the fireball and it homes in on Luigi first if it lifts its head down. If it does not lift its head, the fireball splits in two. This attack can cause the Burn status ailment.
The Shroob Rex's final attack is simply charging into a Mario Bro., running off the battlefield and into the back of the second Mario Bro. This attack can be dodged or countered by jumping, but only if certain prerequisites are met. A normal-sized Shroob Rex is too tall to jump over, so the attack can't be dodged. However, whenever a Shroob Rex is jumped on by a Mario Bro., it becomes flatter like a normal Rex. Only the flattened Shroob Rex can be jumped over.
At 200 HP, Shroob Rex has the highest HP of all regular enemies, surpassing the first four bosses and, in the international releases, on par with Swiggler.
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