You know in a perfect world (say, the rift) where you can choose who you fight and pick your champion, that's great, but tanks are GREATLY overpowered in the Howling Abyss, especially if you are on a team composed entirely of mages and the enemy team has even just 2 or 3 tanks. It's virtually impossible to win in a situation like that, because like they said, mages don't have ANY real options when it comes to dealing with High-health foes.
What Riot could do is make an item that is similar to Warmog's Armour, except that it deals percent health damage only if the target's max health is over a certain threshold, maybe 3000. Nobody wants to have an insta-kill item. What they want is to be able to deal damage enough to kill these massive tanks without having to have 5 people against the one. The only mage who has anything close to that is Veigar because of his essentially limitless potential for AP stacking, which actually just amounts to MASSIVE damage (more than any other mage can put out).