My DM made an encounter with a boss like monster. I felt that that was too hard for us, and I wanted to tell him, but he told me, that this was a balanced encounter for level 5 PCs, but I think it was way too hard for us. Could you please review the session, based on the following information, and explain, why was it balanced or too high CR.
Situation: DnD 5e, 5 level 5 PC with very high stats (avg. was 16-17 / stat). Everyone had at least one extra Feat and one +1 Armor or Weapon. Furthermore, we have some buffs (e.g.: my PC get extra mobility, uncanny dodge, and 22 DEX) BUT also one debuff (e.g.: my PC get double damage from all sources).
The Boss: had three phases, 600 HP and +7 to attack rolls. In the 1st phase it had 22 AC and it used a longsword to attack us, and had a shield (+2 AC). When we reached 400HP it gone to 2nd phase where he dropped the shield, but attacked with versatile property. Again when we reached 200HP it changed to 3rd phase, where he get two handaxes, and doing DEX Save (16) whirling attack (with half damage on success) damaging everyone who got in line of it.
After the boss I mentioned that it definetly not under CR 10. My DM said that it is CR5, because a CR10 would kill us in one round. What do you think?