Why would you? If you have initiative, seems better to time stop, ice storm, true strike followed by meteor storm. Or just pound out lots of damaging long range spells and wait for your opponent to miss a save: delayed blast fireball, horrid wilting and so on.
If you're dead set on the hip throw, cast time stop, action shove. If you don't, can always cast Chain Contingencies with a teleport any time they get close to taking you down, then it doesn't matter if you don't have initiative. Not sure why you let them get the underhook when any 20th level wizard has access to fly, but these things happen I guess.
To not be an asshole, yes. I'll take a bladed stance against the underhook, and try to pressure straight down to the ground, to a point in between us, with my overhook (don't push into them, as you'll get suplexed, and don't pull away, or single legs/other nightmares await). Once I've neutralized their underhook, I'll try and step in front and hit the hip throw. Lower body movement/mechanics for the hip throw are, as far as I understand it, the same as they would be in a Judo/Sambo/Whatever situation. The trickery of overhook pressure isn't as intuitive though.