Gallivant
"Well at least you people aren't like that one famous philanthropist who died while choking in a taco bell."
The lady offers the group a sympathetic look as she rapidly files through her cheap plastic clipboard. Pages stop passing by once she reaches a yellow disclaimer form. Ripping it out soundlessly she hands it to Duo for the three of them and gives each individual a pen saying "It's always sunny in Elysium, U.S.A".
"Before you can really find out the challenge I will need you all to sign these disclaimers. You know just in case a wee accident happens. Our legal time is kind of overbooked right now so we can't risk anymore lawsuits and the like." She grins up at the group with thoughtful pen in mouth and mumbles out. "Name is Becky by the way. Pleased to meet all of you newly deceased."
The disclaimer is an eye hurting yellow form that has a whole bunch of little script on it listening all the possible dangers of taking a challenge. Dangers include things like internal bleeding, external bleeding, general eternal bleeding, mad dog syndrome, personification, midas touch, mesothelioma, and a rare case of poetitis. At the bottom are three simple lines waiting for group signatures to fully place on responsibility on them.
A Super Thought Exercise
I would consider the ability to cancel out one human sense in one individual at a time to be most effective. As in the power to cancel out one sense (touch, smell, sight, hearing, or taste) at a time for one individual.
With such an ability you could attend the events of a world leader, business CEO, or military head to begin the madness. A snap of the fingers could easily render said person with the inability to see or feel pain and that would begin the questioning. About the fitness of the individual and if you keep doing this then they make soon experience sensory deprivation. Choose the right people and you could bring a state to its knees with the clear absence of leaders. Now as for taking the world you go after the people who have the keys to WMD (weapons of mass destruction) facilities. Make them unable to carry out the position and render the staff in disarray. Best part is no one would realize what is happening and you could take all the time you need to do it. Or play it like a fun game of conquest and strategically take out.
Shadow ruler is the preferred for this.