The Balrog in FOTR block faces 2 problems too often: skirmish cancelling and
Armor /
Gimli's Helm. To counter skirmish cancelling events you can pack 2x+
Relentless, but that's useful for
Flame of Udun only since
Durin's Bane fights by himself; also, that's only for events, so does nothing against
Filibert Bolger and
Gandalf's Staff. Instead, he can rely on
Saruman's Snows to assure the skirmish will end in wounds or death.
To counter
Armor and
Gimli's Helm, you can use the
Whip of Many Thongs at site 5, or
Beyond the Height of Men /
Isengard Smith at previous sites. But
Saruman's Snows blocks your whip, and since requires to exert an
minion the
tricks will be better.
To increase the Balrog's strength, besides weapons, you can pack 4x
Dark Places in a deck with very good cycling. To reduce strength on FP characters,
Goblin Scrabbler can get rid of those irksome Last Alliances while
Isengard Smith can do so with weapons.
Goblin Warrior can deal with Thrarin (he also removes
Endurance of Dwarves' tanking).
Horn of Boromir can be a big problem though, and the only efficient ways to remove him I know are 2x+
Frenzy (with bulky Goblins, unique ones with
Dark Places or
Moria Axe are very good for such task) and
Saruman SotE (with a strong, preferrably fierce dmg+1
minion), also at a different site than
Durin's Bane's.
That's if you want to build your deck around The Balrog. But if you just want to splash him, forget all I said above;
Durin's Bane is awesome in
grind or
weather decks. He can also be great in corruption decks, 2x copies of him with 4x
Must Do Without Hope and the highest cycling possible.