How do I do a 720 backflip with a grab?

You need a character with a high spin stat (I would say over 50). Try and make a big jump on the halfpipe lip just as youve passed the last of the double halfpipe section, just as it starts getting really steep (since you'll get more air there). Make sure to not do any jumps for at least 5 to 10 seconds before that and simply focus on getting as much speed as you can by pressing up on the joystick (since this will give you more air). Make sure to jump just before reaching the lip of the halfpipe for more air. Start by doing the backflip (prep the backflip so you flip faster by holding down the jump button and down on the joystick before releasing the jump) and once the backflip is completed, immediatly switch to the spin. (I say this since it's often hard to start doing flips in halfpipes once youve started doing spins)
Follow these rules and you should be able to complete that objective without too much trouble