Use a green frequency laser.
Or trained dolphins.
Armored Orcs ? On fire ? To confuse both radar and infrared. I have a vison of a Monty Python sketch type scene :
Orc, with British accent : “But I don’t wanna die !”
Witch King, with British accent : “None of that, now ! You wanted to join the Mordorian Air Force; well, here you are !”
- Witch King looks down *
Witch King : “Alright ! Off you go then !”
- flings flaming orc off Fell Beast *
Orc : "Ahhh ! "
Witch King, calling downward : “Remember ! Aim for the front of the damned thing !”
As to the destruction of the One Ring, Gandalf his own self said that no dragon fire, not even that of Ancalagon the Black (who was overthrown by Earendil) was hot enough to consume the One. The requirement indeed was not for sufficient heat, but for fires of Mt. Doom, where it was made.
I do not recall JRRT ever making clear why this was so (it is not in Letters, but may be tucked away somewhere in HOMES) but I would hypothesize that it is because it is at Sammath Naur that Sauron poured much of his own native power into the ring, at its forging. This he did to expand his own mastery over the creatures of the world, and the world itself.