Saeviomagy
Adventurer
I'd lower damage to 1d4+6 and subtract 2 from his attack rolls, but that doesn't mean the fight's over.
Soo.... you've got an at-will that applies a -2 attack and damage penalty for the entire rest of the encounter, and you think that's somehow balanced? Riiiiight...
As for trip not being unbalanced: it's less imbalanced than 3e, but it's still pretty bad. The at-will polearm trippers are already causing DMs problems. For many melee NPCs (and with the right tactics from players), being tripped is crippling. The hand of bane for instance will be unable to use flail of tyranny unless his foes choose to end their turns next to him, and he will be unable to attack if his foes are smart enough to end their turns one square away. And it's the smart players you have to balance against.
As for why not make it "balanced"? Because the benefits are so good that they would have to almost never work. And powers that almost never work are not fun (see "god call" above).