The encounter you describe is extremely deadly.
However, Hoard of the Dragon Queen is not designed as a series of static encounters. Rather, the book generally describes adventure areas and the behaviors of their inhabitants. To quote designer Steve Winter:
We didn't want to script NPCs' locations or motions. Guidance is given in certain cases; where nothing is said, it's up to the DM. ... is meant to be a site in motion, not a store window display where everything and everyone is frozen in place until PCs come to attack them.
The adventure presents several situations that could be "extremely deadly", and expects players to use discretion when choosing their battles. This could mean finding a way to balance the odds, or finding a way to achieve their goals without resorting to bloodshed at all.
That said, the encounter you describe is not in the book. I see two likely possibilities here:
- Your DM is running the adventure as intended. This style of play can be jarring for a group that is only used to facing well-balanced, challenging-but-winnable combat encounters. If this is the case, you should have an open discussion with your group about which style of play is right for you.
- Your DM scaled up the "boss battle" to make it more epic but still winnable, and just overdid it. In this case, your DM should follow the encounter-building guidance on page 82 of the Dungeon Master's Guide to avoid overdoing it again.
Anecdotally, when I ran this encounter it was deadly without any guard drakes:
The party was four 3rd-level characters, and Mondath was encountered with just a few guards. The party knocked out all the guards, but two of them fell unconscious in the process. In the next round Mondath cast Mass Healing Word on her allies and Hold Person on the third PC, who watched frozen in horror as the fourth PC threw down her bow and pledged her life to the Queen of Dragons. It was incredibly entertaining for all, despite the total failure. Things got off track for a bit, but the party came back later and ambushed her alone in her sleep. That was a much easier encounter, despite still being level 3.