The trick for this minigame is to try to count down to zero slightly quicker than the actual timer without blowing yourself up. That does take a bit of practice mind you.
Warning! - Prolonged play of certain minigames in Wii Party U - such as this one, can lead to muscle pain in the arms. i.e. Wii-itis. As I found out the hard way. But I did manage a personal best through all the hard work however.