IWHBYD is pretty evil to get if you don't know the secret. Sad thing is, it went through my mind to jump through the rings when I was on that level, just not in any order, I was just going to jump through them 1-7, not the 4xxxxx order.
Some of the skulls were in evil places, others were somewhat easy to find. I found two on accident by taking the wrong turn, had to look the rest up, like this one which is on this four level platform when you walk down some stairs, like, wtf, why would you check there.
Really, I only found 2 by myself, had to lookup the others. I found Thunderstorm, by flying to the wrong platform, and the one on the last level "Halo", because I took a wrong turn.
The grunt party one is pretty funny though, yay!
Theres also one where a flood is holding it and you have to shoot him down, I didn't even notice that guy when I was playing that level. Would have never found that one.
But, the Hayabusa Armor/Helmet is pretty sweet.