Balrog Tatics

[Alatáriël Lossëhelin]

'Long I fell, and he fell with me. His fire was about me. I was burned. Then we plunged into the deep water and all was dark. Cold it was as the tide of death: almost it froze my heart.'
'Thither I came at last, to the uttermost foundations of stone. He was with me still. His fire was quenched, but now he was a thing of slime, stronger than a strangling snake.

'There upon Celebdil was a lonely window in the snow, and before it lay a narrow space, a dizzy eyrie above the mists of the world. The sun shone fiercely there, but all below was wrapped in cloud. Out he sprang, and even as I came behind, he burst into new flame. '

-Gandalf's account of his battle with Durin's Bane

There's a part missing between the 2nd & 3rd paragraphs. Sorry this isn't an exact quote as I don't have my text with me, but Gandalf says something to the effect that "ever he clutched me and ever I hewed him" and I followed him from the bottommost cavern to the topmost tower by way of the Endless Stair by holding to the Balrog's heel. The "ever I hewed him" implies to me that Gandalf is using Glamdring to smite the Balrog. So it appears that at least one sword is effective on Balrogs. I'm sure that the power of his ring is also being used, although Tolkein does not specifically state this.