From the Super Mario Wiki
Hammer Uppercut is a special move performed by Giant Luigi in Mario & Luigi: Dream Team. It is first used against Mount Pajamaja, and the move icon is a hammer pointed vertically with a vertical arrow on top of it. In the hammer section of the attack menu, it is next to the Hammer Chop icon.
When it is used, Giant Luigi runs up to the enemy and reels his hammer back. The player must slide the stylus upwards on the Touch Screen to make Giant Luigi swing his hammer upwards into the enemy. The faster the player slides the stylus, the more damage that is dealt and the greater the rating the player receives, ranging from the "OK!" rating to the "Excellent!" rating. Similar to the Hammer Chop, the Hammer Uppercut has the secondary effect of knocking the enemy backwards, which makes the Hammer Uppercut key to defeating Mount Pajamaja, Earthwake and Giant Bowser in their respective battles.
Slide fast for a harder hit! (Smack the foe backwards!)
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