If played as written it isn't so bad. It's certainly not easy and I've usually had someone go down during the fight. But no TPKs.
The dragon doesn’t try to catch all the heroes with this attack, but focuses on just the nearest opponent. Once the dragon uses the breath weapon ability, they can’t use it again for 1d4 rounds or until they get a critical hit on a Strike.
The dragon can't use draconic frenzy and the breath weapon in the same round. So more likely it will breath weapon and blast one PC then strike, possibly recharging the breath weapon. The second round it could repeat or do draconic frenzy.
I have the dragon using all its abilities, so I tend to fit in a draconic frenzy rather than just repeatedly breath weapon. It will make three attacks, at best at +14, +7, +6 or thereabouts. Using all its abilities seems intended, meaning the breath weapon will be available every other round.