From the Super Mario Wiki
Lethal Bob-ombs are variants of Bob-ombs that appear in the cellar of Shroob Castle in Mario & Luigi: Partners in Time. Lethal Bob-ombs' fuses are always lit. If they touch a playable character, they explode. Additionally, they are constantly flashing, wear purple shoes like Rob-ombs, and are faster than Bob-ombs. They can only be fought by Baby Mario and Baby Luigi. They are the last regular enemy introduced in the game.
In battle, Lethal Bob-ombs attack by approaching the baby Mario Bros., and then run back and forth in front of them. It then rams into one of the Baby Bros., self-destructing and causing damage. The attack can be countered using Hammers.
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