Holy crap, this was bad.
First of all, a giant mecha. Really? A giant mecha? My only guess is that they want to buy their way into the Gundam fans. And it's not even an effective shape. Apart from this: "OMG, a big METAL thing, what shall we do?" ask the metalbender and the lavabender. Maybe open a pool of lava beneath that army, end of story? If only lava would melt anything in minutes...
We saw truly powerful benders fighting side by side and against Aang - have people forgotten bending, or what? Where are the master benders, like Bumi, Jeong Jeong, Master Pakku, Iroh?
The whole Legend of Korra is sad. It's depressing, it's turning a beautiful, strange, unique world inside out; giving us a realistic "show". On top of this, Season 4 is predictable and boring.
Can I please have the real Avatar universe back?