Because the Valesk's toughness is only 2, and the Mauler has trample, which means 2 of its 4 damage gets dealt to the Valesk, and 2 gets dealt to the opponent
In the film Yu-Gi-Oh!: The Dark Side of Dimensions , an exclusive form of Summoning known as Dimension Summoning is featured, which allows players to Summon a monster with ATK or DEF lowered from their original values, and receive damage equal to those when the monster is destroyed instead of normal battle damage
And if you have cards that make many faerie tokens like Alela, Cunning Conqueror, Bitterblossom, or Oona, Queen of the Fae, Obyra can do a lot of work
This strategy aims to Summon 1 of each Valkyrie from the hand with Ride of the Valkyries, use their abilities to destroy your opponents best monster and copy its ATK, then win the Duel by attacking with all your Valkyries twice without giving your opponent a turn in between, courtesy of Mischief of the Time Goddess
Sealed in wrapper