Hm, it is always good to see artist that don't scram around when getting a critique xD
I'm a bit cautious if i ever give critique nowdays, since I got some really stupid and harsh replys on DA when I posted a critique about their works ^^;
About the things you asked:
The Seraphim is O.K. so we discussed it I think
The Duel: lets see....
You were right about the head, it is slightly smaller than it should be.
Her right hand at the elbow is a bit more bent outwards than it should be for that position.
I guess most of the anatomy issues are mentioned already, plus if you make the perspective right, they are all but disappear. Since most of them come out only because of the angle.
Anyway, i know that she is a Sister Repentia

Still I think that chainsword is a bit bigger than even on the models

Just shorten it a bit, not too much than it is ok i guess ^^
The Eldar:
the pointing arm is strange, since it looks like he has no lower arm xD Or at least really small and thin compared to the rest of the arm and body. I know it is funny, but try to use your OWN body as reference. It is there, in reach, and you can always figure out how things work if you look at your own hand in the desired pose that how it will look like ?
Ok, some pose would need a mirror to work, othervise you would hurt with the twisting xD
The neck part is nice, but in the other hand, off. You streched out the neck part a bit, making the collarbone to be lower than it whould be + the head is higher too a bit. Nothing wrong with the neck muscles, you done them nicely ^^
I looked though your gallery in DA too by the way ^^
Nice, you really improved a lot :3
You are a bit ahead of me, since I need to switch to computer art. The traditional style is nice and stuff, but a "bit" slow ^^;
Nowdays I need 30-40 hours to finish an A/3 size totally detailed picture

Not good xD
If you are planning to make pictures that use dynamic poses and angles, I highly suggest that you make some sketches on some paper so you can try out how it would look like in the end. Another thing that you can see how the angle works or the composition itself. It is really fast if you do it in a sketchy style in a small size.
I have always done the same so far in my last couple of works and it payed off ^^ And you can counter the anatomy issues if you learn how to draw the different body parts, like they were belonging to a puppet:
Try to brake down the body to simple objects when you do sketches. That way you can do some speed-sketch (thus saving time) and you can even take a good look how it will look like (raughly though xD).
I always do such "thinking-sketch" when I plan my drawing. And you can still think about the details on a sketch paper. When you finalise it on the computer with the tablet, you will have an easier time, since you will have a "reference" of the pose right next to you and don't need to figure out it on the screen. And good for practice anyways
Anyway, if you are curious how I work, I'm on DA too with the name of:
tanathiel.deviantart.com