From the Super Mario Wiki
The Lethal Bob-omb is a more powerful variety of a Bob-omb found in the cellar of Shroob Castle in Mario & Luigi: Partners in Time. A Lethal Bob-omb's fuse is always lit, meaning once it touches someone, it explodes. Additionally, they are constantly flashing, wear purple shoes (like Rob-ombs), and move more quickly than Bob-ombs.
In battle, Lethal Bob-ombs attack by approaching the Baby Mario Bros. (as they only appear in areas accessible by the baby pair), then running back and forth in front of them. Eventually, it chooses a Baby Bro. to attack and rams into him, self-destructing and causing damage. The Baby Mario Bros. can counter this attack with their Hammers.
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