Effect
Sherry's Hyper is Whimsical Windmill. When used, the player will initiate a battle with all other non-KO'd units in succession. In these battles, the player will gain +1 EVD and will be on the defending side. Players will be fought in order (Player 1, then Player 2, then Player 3, then Player 4).
Strategy
Whimsical Windmill is a somewhat lackluster Hyper card that is difficult to use effectively. However, when timed correctly and with the proper circumstances, the card can be quite powerful. Given that the card requires the player to fight all opponents in a single turn, it has a very limited window of ideal use, as facing all opponents is likely to be met with heavy retaliation. As a result, the player will want to activate the card when all other opponents are nearly KO'd, or when many of them are KO'd so the player can single out 1 or 2 opponents. While scenarios where this is possible are somewhat limited, there are multiple cases that may cause such an opportunity to arise. For instance, if multiple opponents land on 
Boss panels, they will likely be gravely wounded and some will likely be KO'd. Or in another potential scenario, multiple opponents may be at low HP due to cards such as 
Invisible Bomb or 
Sealed Guardian. Another tactic the player can use is to increase their prolonged battle endurance with the aid of battle cards such as 
Quick Restoration and 
Rainbow-Colored Circle. Due to going second, the player can also skip certain opponents by using 
Shield Counter or 
Tactical Retreat in order to get to the opponent they really want to fight. Given the restrictive nature of the Hyper, the player may want to bring at least 1 copy of 
Mimic with them to receive a broader-use Hyper, and opponents are unlikely to copy Whimsical Windmill.
The main flaw of Whimsical Windmill is its very design. While a well-timed Whimsical Windmill has the potential to KO all opponents, if the player miscalculates or consistently rolls poorly, they more than likely will end up KO'd. The Hyper has limited use and is also rather expensive at a base price of 30 stars and a level requirement of 3. Consequently, great care should be taken to ensure that the reward justifies the risks of using the card. The card also boosts the EVD stat but does not improve either the ATK or DEF stats which are generally more critical for prolonged fights, especially when the opponent attacks first. Likewise, since the opponent attacks first they will be able to use their own battle cards to try and KO the player. An unexpected 
Accel Hyper or 
Final Battle from an opponent can quickly spell certain defeat for the player. The last and most inherent flaw is that depending on the matchup, an opportunity to use the Hyper may never occur at all or may be too dangerous depending on the Hyper cards used by the opponents.