So, I've taken this as an incentive to finally finish my preceding playthrough which I had stopped just before starting the attack on the armada (I skipped finishing the big mindreading quest because since it's been so long and I don't remember who I'd still have to mindread in order to complete it, even though I have solved this quest before).
Indeed, lowering the difficulty won't help you much if you use the wrong strategy
. Flying far ahead indeed was the way to go once again - however, on the Nightmare difficulty, that alone won't suffice.
Generally speaking, liberating Berlihn will make the armada quest slightly easier, but the final fight more difficult (obviously, because you'll have to face Berlihn and Ygerna rather than just her). If you choose to side with Bellegar, it will be the other way round, even though Bellegar's weapon, as far as I remember, isn't that much worse than Berlihn's "
eye of the patriarch".
Correct use of this skill is vital! Since the ire of the patriarch destroys the building it hits plus everything around it, only use it on major platforms. Be as efficient as possible with the cooldown here; as soon as this skill becomes available again, stop whatever you're currently doing and quickly find a new huge platform to put it to use.
Do not use Dragon's Wrath, it's just an inferior version of the patriarch weapon, because you have to get really close to the targets and the radius is very small, whereas you can use the patriarch weapon across a much larger distance.
Do not use the regular flamebreath either (unless for doing the last bits of damage to destroy a fortification). It takes both too much time and mana;
two flame spheres are perfect to destroy a ballista, and the cooldown is much faster than your mana regeneration after regular flamebreathing.
Most of the flying enemies can also be taken out by a single flame sphere. So occasionally shoot one at them whenever you come across one; since the flame sphere is homing, you usually don't have to worry about that foe afterwards anymore.
Otherwise, I summoned a Wyvern whenever possible, but didn't pay too much attention to it. Also, I occasionally switched on the Dragon Shield, but be prepared to swallow a lot of potions anyway.
Keep in mind you can combine a healing plus a revival potion, since the latter count as mana potions. This way, you can heal around 500 HP simultaneously, rather than just the 300ish the largest healing potion grants you.
Most importantly, keep the distance between you and the zeppelin as large as possible. You don't want it to catch up with you while there's still enemies around who are attacking you, because these will then consequently also go for the ship.
As long as you make sure to leave no building behind, you won't have to worry about anything in these further back areas where you have already been.
Even if you miss an occasional ballista or flying enemy, the zeppelin can handle a couple of those. Be sure to take out breeding nests and lightning towers above all else, and prioritise those targets close to the designated route of the ship (keep looking for the golden arrows; there's no need to clear buildings which are at the outer rim of the fortress).
The better you protect the zeppelin during the early stages, the higher its chances of survival during the final phase. I quickly decided to restart from Aleroth once, because I did fail on my first attempt and quickly realised it was down to the zeppelin having taken too much damage already, making it impossible to survive the final stage where it's supposed to crash into the fortress.
During that final stage, the ship will inevitably take damage, but during the earlier phases, you can keep it almost completely undamaged. If you manage to do that, then the damage it is bound to take during the final phase will not be enough to destroy it.
I was actually surprised how quickly the cutscene kicked in, just a couple of seconds after the captain had announced he would crash the ship into the fortress. I barely had time to destroy another lightning tower before the quest was already completed
.
) but I vaguely recall it being a bad thing. Not in a crashy sort of way but at risk of failing the mission.