From the Super Mario Wiki
The Wonder Thwack is the most powerful variety of Thwack. They are found at the Star Shrine in Mario & Luigi: Partners in Time. Unlike regular Thwacks, which come in pink and green, Wonder Thwacks come in red and blue.
In battle, Wonder Thwacks use the same attacks as regular Thwacks, but they move faster. Even though they are more powerful than normal Thwacks, Wonder Thwacks still have the same amount of HP. Before each attack, a Wonder Thwack twirls and either remains the same color or switches to a different color. If red, it attacks Mario; if blue, it attacks Luigi. After twirling, the Wonder Thwack attempts to crush the appropriate Mario brother. However, since they fly higher than normal Thwacks, the Baby Mario Bros. must wait longer until the Wonder Thwack drops within range before countering with their Hammers. Wonder Thwacks can also fly low and ram into a Mario brother. The bros. can dodge or counter this attack by jumping. Additionally, as with their weaker counterparts, they can summon more of their kind to join the battle. Wonder Thwacks can also turn yellow after twirling and attempt a final resort attack. After turning yellow, they split into two yellow boulders. Each boulder rolls towards a Mario brother. Mario and Luigi can dodge this attack by jumping. Red Wonder Thwacks are resilient to Bro Flowers and Mix Flowers, while blue ones are resilient to Ice Flowers. Each color receives more damage from items that its counterpart is strong against.
Unlike the Thwacks, the Wonder Thwacks do not group to form a Thwack Totem.
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