Is this assuming the Roc just swings down and fights toe to toe with the party to the death? Because that is not going to happen.
The Roc would fly down, Hit/grapple with Talons for an average of 23 damage, his beak does another 27 damage on average. If he flies up 120ft and drops the PC, that's another 42 damage on average. That math means, on average, instant death for the Cleric or Fighter, and would KO the Monk even after the slow fall damage was reduced.
The PCs best case scenario for damage is for the Roc to pick the Cleric as a meal and when it lands, the Fighter and Monk go all out on it before it can fly away. The fighter could do 5d8+30 at best or 52.5 damage on average with it's action surge and the Monk could do 2d8+2d6+22 or 38 damage on average with a Ki. That's only a little over a 3rd of the Roc's hit points and 1/3 of the party is dead lol. I guess I would allow the party to all get off one ranged shot against the Roc while it descends. That would help a little towards the goal of knocking off half it's hp. I would for sure make it fly away if the party manages to knock it to half.