The hidden boss in Despair Dungeon: Monokuma's Test, Monokuma V3, is located on the 100th floor of the dungeon. You will be able to fight him right after beating Monokoppa V3 on the same floor. Check Bronze Reaching the Summit for more information on how to reach the hidden boss.
He is the hardest boss in the game and will require you to have a party full of U-Rank characters in order to succeed. You should also be using skills that are maxed out. It's a good idea to have two Sports-type characters and two Variety-type characters in your party. Use your Sports-type characters to use Falcon Kick. You can buff up their attacks with Zeal, Skill Power Up, and Awaken Up. Have your two Variety-type characters to use Wind Barrier to nullify Monokuma V3's Wind Jet All. Make sure your Variety-type characters know Recover Influence (having it maxed out will allow all of your party members to be healed in one move), Fighting Spirit so that you can revive your party members, and Cure Condition to remove ailment effects. Monokuma V3 can use Confuse on your entire party, causing your party members to attack each other. Using Cure Condition during those situations would help. Using Ultra Defense would also be a very good countermeasure for Confuse.