Hey man,
meat and dodeqqa make some good points and this is more just reinforcing what they've said really.
1, Like Dodeqqa said the lighting is very confused at the min and I'd say there's probably a bit too much white too. It looks like he's giving off light as apposed to it bouncing off him. Try and consider where your light is coming from, almost like the director of a film, and imagine how that light would effect your subject matter. For this paint over I chose a direct over the head light as I figured that was what you were going for.
Try to Keep a nice contrast, between lights and darks, like ive done on the top of his head if you want to describe forms better and make your dude pop.
2, The lighting on some of your forms seem a bit off and its making the head seem a little flat in places (like around that lip and cheek.) Still life studies with a lamp are great practice for this man and what dodeqqa did with the lines wrapping around the forms on the top image is a great way to imagine the thing your painting in 3D. Oh and if your struggling with forms try painting in black and white, it'll make life a lot easier, that's what I did here with your paint over ;)
3, Your interpretation of nemesis is cool, I like the overly large teeth. I remember seeing this dude back when I had a ps1 and shitting my pants. Now the trick, as a concept artist and illustrator is to take those things you think are cool and push them to a way which is unique to you. What is it that makes this dude so creepy? How could I make him scarier? That's basically what I did with his teeth here just to kinda show a point, what could be done, from a design perspective to make this better? If you saw this dude is a video game today, would you be scared? A bit of a random point but just something to bear in mind ;)
4, Be sure to study up on your textures mate, I can see you've already got some in there which is awesome but making him all gross looking and slimy would take this dude to a whole new level. Try finding things in real life (slugs maybe?) and paint them to work out how to capture that texture.
That's about it mate, you've got a great piece here with loads of potential and I hope this helps in some way. Sorry if it came across harsh in anyway or ive overloaded you with pointers, that was never my intention, I just want to help in the best way I can. All the best buddy.