I did, however, decide to magnetize the rider (for obvious reasons) and the wings. I wanted to magnetize the wings because the Terrorgheist's wings basically double the model's height and look like they could easily get damaged either during play or during transportation. One of the best things about magnetized pieces is that they can break away if they get too roughly jostled.
1) Set the magnets in the shoulder musculature before gluing the muscles to the rest of the Terrorgheist.
2) I would recommend magnetizing the wings to the claws and gluing the claws in their appropriate places. I did not have small enough magnets for that job, so for the right wing (clutching the small rock) I glued the claw to the wing and magnetized the rock to the ground. This puts a lot of stress on the wing/claw joint there, so make sure you attach the claw very solidly.
3) It would be pretty tough to paint the basing after gluing the Terrorgheist down, so I recommend blocking out space for the feet, then doing the basing, and then gluing the feet down after you're totally finished with the base. I gave the feet plenty of space on the base when I did the painted basing, then filled in the sand-free area with static grass so that it wouldn't look out of place.
4) Paint can get chipped pretty easily anywhere there's a magnet connection. I would recommend coating the magnet on the Terrorgheist's back (where the Ghoul King attaches) with something to help it not chip. I used Liquitex matte gel base (because I have it), but you could probably alternatively put a really thin layer of Green Stuff over it to similar effect.