Hey there Tyffany,
You are a little confused on "what" you need to equip. It is NOT the Atk 15% equipment that you need to put on. What you need is Atk% as the Primary stat. Look at the yellow circle here...
The reason that weapon, helmet, and shield are not included in the mission is because their primary stats are always static at ATK, HP, and DEF. However for gloves, chest, and boots, their primary stats can be different such as Crit Rate, Crit Dmg, HP, HP%, Atk, Atk %... (ect.). So, the Primary stat has to be Atk #% and NOT just Atk #.
I hope that helps.