Here is why the banner is not broken.
1) elves are easy to kill. They are T3. The only real anvil in the army is the frostheart phoenix and it can't be given ranks/ horded up. Phoenix guard is a soft anvil because of their 4+ ward, but eventually they die too.
Nothing with a 2+ ward save is easy to kill.
Also, it depends on whether you're attacking them at range or in combat. If you only have combat stuff, then your unit first has to survive all the elf attacks, and then do enough damage back to win combat, or at least not lose it too badly.
2) most anvils are T5 or higher, or have a 2+ armor save along with some staying power (IE wounds, T4, etc). Plaugebearers with full regeneration, deep enough ranks and support from lore attribute of nurgle can almost be called an anvil. Elves do not really have a unit that can do this. Maybe dragon princes, but their vulnerable to a number of anti-armor options and small in number.
T5 can be mitigated by strength-improving weapons, poison, rerolls to-wound, buffs that increase strength, hexes that reduce enemy toughness etc.
Likewise, armour is easily weakened, and even 1-2pts of reduction make it far less painful to face.
Ward saves are mitigated by... virtually nothing. Strength doesn't matter, Armour Piercing doesn't matter, there are no hexes that reduce ward saves, nor any buffs that allow a unit to reduce enemy ward saves.
If the banner gave the unit T5, or a 2+ armour save, I'd have no problem with it. Instead, it gives them the best save in the game, and the only one that's impossible to mitigate.
3) banner of the world dragon shifts the high T roll to the ward save roll. So instead of being a 4+ to hit, 6+ to wound and a 4+ save (like beasts of nurgle or crypt horrors often are), the elf unit is going to be 4+ to hit, 4 or 3+ to wound and then a 2+ to save against magic attacks and spells.
See above - there are many ways to mitigate high toughness, but virtually nothing that can be done about a 2+ ward.
4) the high elf unit carrying this banner is completely vulnerable to most armies attacks. The banner does nothing for them than mitigate miscast damage. If their wizard is bunkering in the unit, now the unit is vulnerable to getting him killed if it engages, this means the banner involves risk when used in both an assaulting unit and a bunkering unit.
Surely you're not going to be bunkering a mage in that unit unless you're against daemons though?
In any case, there are a lot of banners that aren't always useful - but most of them do a hell of a lot less than the banner of the world dragon, and cost a lot more.
5) the unit is completely vulnerable to hexes and test based spells. It is completely vulnerable to flanks and rear charges. If engaged on two or more fronts, that unit can't bring its martial prowess to bear. If disrupted, that unit has lost most of its fighting power--no ranks fighting, no bonus.
Well, I think the definition of 'vulnerable' is arguable. I mean, half of the current test-or-die revolve around initiative tests, so the unit's "vulnerability" is looking somewhat dodgy at this point.
Furthermore, it's no more vulnerable than it would be without the banner. It's not like the banner imposes a penalty to characteristic tests or any such. Nor is there anything the unit could take instead to mitigate such risks - the world dragon banner is the absolute best it can get for defence.
All this banner does is make that unit an anvil against daemons and magic wielding characters. To defeat it, just bookend the unit with two of the heavier choices and throw a hex on it, it should collapse in one turn and you chase it down. Its not obvious or point and click, but that unit is killable.
It depends what you have available in your army. What if your "heavy" units have magic attacks, or can't actually beat the banner unit in combat? Elves might have low toughness, but they're striking before you, fight in an extra rank and will almost always have rerolls to hit.
Also, why do you keep calling it an anvil? If the banner is put on an offensive unit e.g. Dragon Princes, they don't suddenly become an anvil - they're still a hammer unit that now had ridiculous defence against damaging spells, magic shooting attacks and magic combat attacks.
