The Elder Shrooboid is a boss in Mario & Luigi: Partners in Time. He acts as the Boss of the Star Shrine and the game's fifth major boss and is also the holder of the fifth Cobalt Star shard. It is a red Shroob with yellow spots and spikes on top of its head. It is the strongest of all the Shrooboids. Mario, Luigi, Baby Mario and Baby Luigi must fight this powerful Shroob at the Star Shrine high above the Mushroom Kingdom. Before the battle, the Elder Shrooboid defeats Toadbert and Kylie Koopa, and steals the Toad's Cobalt Star shard. When the Mario Bros. arrive at the top of the shrine, Toadbert makes an attempt to reveal the truth of the Cobalt Star and the Elder Princess Shroob trapped inside by having Mario rub off the dirt on his strange sketch that he had made awhile beforehand, revealing two Princesses. He and Kylie Koopa are then turned into purple Shroob Mushrooms by the Elder Shrooboid. Hoping to gather all the shards and awaken its leader, the Elder Shrooboid attacks the Mario Bros.
When the Mario Bros. and their past selves battle the Elder Shrooboid, he uses a variety of attacks against them. His first attack involves trapping Mario and Luigi in crystals, (like the Shroids) and then grabbing a ball and chain. The brothers need to jump to break their crystal prisons as the Elder Shrooboid spins the ball and chain towards them. However, the spinning causes the Shrooboid to become dizzy, making him drop the metallic ball and fall down. The Elder Shrooboid gets up again, screams in a rage, and angrily knocks the ball into either Mario Bro. and lunges head first into the other Mario Bro. The metallic ball either has an "M" (for Mario) or an "L" (for Luigi) marked on it, showing which Bro. the ball will be used against. As such, the appropriate Mario Bro. should jump over the metallic ball while the other one has to jump over the lunging Shrooboid. If the Mario Bros. cannot break their crystal prison quick enough, they will not be able to avoid the Elder Shrooboid's attack.
Like the Junior Shrooboid, the Elder Shrooboid can also perform a spin attack against Mario or Luigi. The corresponding Baby Bro. can counter this attack with their Hammers. However, the Elder Shrooboid keeps coming back, meaning the Baby Bro. needs to counter multiple times. The Elder Shrooboid can also shoot fireballs (like the Blazing Shroobs) from its hands at the Mario Bros. He may shoot two fireballs at once. Mario and Luigi need to jump to dodge these fireballs. As such, Baby Mario and Baby Luigi have to use their Hammers to deliver damage to the Elder Shrooboid. Although, if either Bro tries to jump on the Elder Shrooboid, the spikes on his head damage them, which limits the types of items they can use.
The Elder Shrooboid also can create a Spiked Ball that it would throw towards a Mario Bro. The ball circles around the bros (in a similar fashion to a Love Bubble or Soul Bubble) until it closes on one of them and strikes them. It moves faster and faster after each orbit. If hit, the Bro. will be damaged and possibly poisoned. The ball can be deflected by a Hammer strike from Baby Mario or Baby Luigi, sending the spiked weapon flying off into the air. Deflecting this attack, the Elder Shrooboid gasps and falls over from shock. When on the ground, the Elder Shrooboid is vulnerable to jump attacks, and any attack deals more damage than usual due to his defense being lowered in the process.
After the bros take off 1750 of his HP, the Elder Shrooboid starts to shrink into nothingness. Surprisingly, the Elder Shrooboid instantly grows to humongous proportions. When in this state, the Elder Shrooboid has the same attacks as before, but the attacks are faster, more powerful, and giant-sized to fit his new body. Additionally, after becoming gigantic, the Elder Shrooboid releases a Shroob UFO from its mouth. This UFO holds a sign with the number four on it, indicating to the Mario Bros. that they only have four turns left to defeat the Elder Shrooboid. If they can't defeat the monster in four more turns, the Shroob UFO fires a massive energy blast that KOs all four of them just like in the battle with the Shroobs. The Mario Bros. can deflect the Elder Shrooboid's spiked balls at the Shroob UFO, resetting the counter back to four turns. When Baby Mario deflects a spiked ball, the weapon flies above the Elder Shrooboid, but if Baby Luigi deflects the ball, the spiked weapon flies above the Mario Bros. Since the Shroob UFO changes positions in the air during the battle, only one of the Baby Bros. can deflect the weapon to reset the counter. If the UFO is above the Elder Shrooboid, Baby Mario must deflect the spiked ball. If above the four Mario Bros., Baby Luigi has to deflect the spiked ball.
Using Ice Flower to defeat Elder Shrooboid is a good idea. As with most Shroob enemies, Elder Shrooboid receives critical damage from Ice Flower. This could also be due to the fact that it is a fire type boss, showed by it's ability to use fire type attacks.
In the Japanese version, the spiked ball always goes up, even if Baby Mario deflects the attack. As such, the Shroob UFO will stay above the Mario Bros. making it much easier to hit in the Japanese version.
After a long battle, the four Mario Bros. defeat the Elder Shrooboid and recover the fifth Cobalt Star shard. Although both their friends are transformed into Shroob Mushrooms, the Bros. have to continue their fight against the Shroobs. With the fifth star shard, the Bros. can gain access to Shroob Castle. Leaving the Shroobified forms of Toadbert and Kylie Koopa behind, the four Mario Bros. must return to Peach's Castle in the future to prepare for their final campaign which is to enter Shroob Castle itself.
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