How you go in room where is last boss even you have heart? You need a full party or a perfect party to open it or ?
for PD apos' room, you need a party of 3 - 6 players who have the heart corresponding to that difficulty to enter.
Party of 4 on PDE, would require all 4 to have Azu's black heart (elite).
You dont need 6 players, 3 is the minimum you are required to enter, for PDN/PDHard/PDE i'd take the minimum as it isn't difficult really.