The 1 VS 100 challenge consists in a serial of 100 short fights in which you are alone (without support characters) and not being able to recover between each fight (except that after beating 10 characters you'll be rewarded a piece of meat). This is a hard challenge which demands a lot of concentration and coordination along with luck. To unlock it, you have to finish the Thousand Arena Battle Royale and One To One challenges in Shin Sekai difficulty (Very hard) 5 times each with different characters.