Dio is a boss in ABJ which you can get to by talking to Halfyummy near left white house.
Dio has 2,000 health. The quick way to take him out is to get JSP (Jotaro's Star Platinum) and use your barrage. When barrage is done, he should be very low. All you need to do then, is to use your strong punch.
He barrages you and sometimes climbs to the top of you, when there he cannot damage you.
Dio's diary: 75%
Chara Knife: 20%
White requiem arrow: 5%
Fixed by Kind guy