My group has just rescued Hekaton and is ready to take down Imyrith. They've fought him twice before. The first time, the PCs (around level 8) figured that the blue dragon died at the Temple of the All Father. So they went to find and loot his lair. They managed to destroy the gargoyle guards and yuan-ti, but were thrashed by the ancient blue. One PC died, and the rest fled for their lives.
In the second battle (prior to rescuing Hekaton), they confronted Imyrith's at the Storm Giant's court. Although the dragon dominated most of the fight due to the power of his terror effect, eventually the storm giants made their will saves and Imyrith was forced to retreat. Even an ancient blue dragon isn't a match against 2+ storm giants and the PCs. Without fear and his breath weapon, an ancient dragon just doesn't do enough damage in combat.
The group has five PCs, who have just reached level 11. One is a Paladin of the Ancients, meaning anyone within 10' is immune to dragon fear. Plus she can cast a Protection from Energy spell. If I follow the advice of the adventure (and provide storm giant allies), the end would be a cakewalk. So I'm thinking that Hekaton would still grant the PCs each a potion of storm giant strength and the claw of wyrm rune (which could give a second PC protection from energy). Given those buffs, I think the PCs would still be victorious, but at least it would be a decent fight. As we're at the end of the adventure (and looking to start a new campaign), this fight will end with either the death of Imyrith or a TPK. Does this sound reasonable?