[Belegorn]

Originally Posted by Aster

1. But Orodruin was hardly "more powerful being" than Sauron, and its flame was hardly hotter than that of Ancalagon... yet it managed to melt One Ring just because "it was forged there"

I think, like Galadriel's mirror, it had something to do with his own work upon it, not necessarily it being able to destroy the Ring because of what it was aside from Sauron's manipulations. Unmake it from whence it was made. No craft less than Sauron's could undo it. When we read of the forges of Sauron it is said of them:

Originally Posted by Rotk, Bk. 6, Mount Doom

He was come to the heart of the realm of Sauron and the forges of his ancient might, greatest in Middle-earth, all other powers were here subdued.

And were Nazgûl really less powerful than the Istari? If so, it is strange that Gandalph, for example, never really attempted to destroy them... You may say that he was forbidden to use greatest part of his power - yet he used it against Balrog...

Yes, but that does not necessarily make them weak beings.

Originally Posted by FotR, Bk. 2, Many Meetings

On foot even Glorfindel and Aragorn together could not withstand all the Nine at once.

Originally Posted by FotR, Bk. 2, Council of Elrond

For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain.

Originally Posted by Gandalf fights the Nine on Weathertop

They drew away from me, for they felt the coming of my anger and they dared not face it while the Sun was in the sky. But they closed round at night, and I was besieged on the hill-top, in the old ring of Amon Sûl. I was hard put to it indeed: such light and flame cannot have been seen on Weathertop since the war-beacons of old. At sunrise I escaped and fled towards the north

From the last quote you will note that they are indeed formidable as a group and that their weakness in the Sun helped Gandalf at one point. However, at night he was hard pressed and escaped them when light came again.

2. Even is airship was much smaller, it is hard to believe that it could withstand massive amount of flame (and Ancalagon was thre greates living flamethrower ever, no doubt) during their loong battle, yet alone penetrate his armour (remember how tough was Glaurung scale). Well... I guess it is more of legend than militray chronicle after all)

I'm not sure if he had the greatest flames of all the dragons ever, although Gandalf did mention him not being able to melt the One, "not even Ancalagon the Black" [FotR, Bk. 1, The Shadow of the Past]. It appears to me that Ancalagon was mentioned specifically as the mightiest of that winged host that issued from Angband in the final assault and not necessarily as the greatest of all the dragons.

Originally Posted by Silmarillion; Of the Voyage of Eärendil

he [Melkor] loosed upon his foes the last desperate assault that he had prepared, and out of the pits of Angband there issued the winged dragons, that had not before been seen <...> Eärendil slew Ancalagon the Black, the mightiest of the dragon-host

You mention the dragon scales. When Glaurung was young he was more vulnerable because his scales were not so tough.

Originally Posted by Silmarillion; The Return of the Noldor

Glaurung, the first of the Urulóki, the fire-drakes of the North, issued from Angband's gates by night. He was yet young and scarce half-grown, for long and slow is the life of the dragons <...> Fingon prince of Hitlum rode against him with archers on horseback <...> Glaurung could not endure their darts, being not yet come to his full armoury, and he fled back to Angband

However, you must remember that Dragons do have a weak underbelly.

Originally Posted by the Hobbit; Inside Information

"I have always understood that dragons were softer underneath, especially in the region of the - er - chest; but doubtless one so fortified has though of that."

<...>

"I am armoured above and below with iron scales and hard gems. No blade can pierce me."

<...>

what he [Bilbo] thought inside was: "Old fool! Why there is a patch in the hollow of his left breast as bare as a snail out of its shell!"

This was displayed by Smaug, and by Glaurung. I think one can assume that Ancalagon was probably hit there.

Originally Posted by Silmarillion; Of the Fifth Battle

with his last stroke Azaghâl drove a knife into his belly, and so wounded him that he fled the field

Originally Posted by Silmarillion; Of Túrin Turambar

he drew Gurthang, and with all the might of his arm, and of his hate, he thrust it into the soft belly of the Worm, even up to the hilts. But when Glaurung felt his death-pang he screamed

Originally Posted by The Hobbit; Fire and Water

The great bow twanged. The black arrow sped straight from the string, straight for the hollow by the left breast where the foreleg was flung wide. In it smote and vanished, barb, shaft and feather, so fierce was its flight.

It can be seen that a good place to get at a dragon would be his belly. However, this is not so easy as it may seem, because two of the 3 guys who wounded a dragon in this way were killed. Túrin was poisoned by Glaurung's blood and died. Azaghâl was struck down by Glaurung before stabbing him, and Bard probably got him from the safest distance with his arrow. Although it seems even their armour was not completely impenetrable as the Dwarves put some hurting on Glaurung with their axes.

No other formidable monsters were created...except Balrogs which could not be "mass produced" though) as well, until it was too late. Hmm.

The Balrogs, like Gandalf, and Sauron were Maia. In the fall of Gondolin they are numerous and less powerful. But then they became lesser and stronger, seven at most.

P.S. Is it mentioned anywhere precisely whether Valar themselves fought in War of Wrath or not?

No, the Elves of Aman [excluding the Teleri who really just lent their ships to the host] lead by Eönwë a Maia.