Hey c_moon, I like how you are using boxes to find different angles and perspectives for the skulls. The thing is, you need to add dimension to the skulls. It looks like because you are using boxes with flat sides, its causing your skulls to be flat. Make sure you think about the shapes of the skull like the ball shape of the mussel holding the teeth, the cheek shape, chin, etc. I did a sketch to show you what I mean, hope it helps :)

Hello @c_moon,honestly most of these heads looks pretty good,you said you had struggle for no4,7,9 but I only see problem with the perspective on no 4 and 9.On 4 specifically I think you're underuestamiting how much skull is shaped like an oval from atop and as for no 9 I think you should made the hole of the nose a bit more squashed since we're looking from below (also here's some overpainting if that helps) On the topic of wanting to draw stylized anime head,I recommend (if you haven't seen it) watching this video by Ethan Becker (https://www.youtube.com/watch?v=YVwCDv5_ahg) where he recommend to simplify the face and learning to draw expression on the face and learning how other anime-esque artist you like simplifying the face (he also has some good tip about drawing faces too) BUt anyway,good job,really love how you're posting consistently!(This is how you get good quick)Keep going,you're doing great!