There's nothing wrong with using MSPaint for pixelling. A lot of great pixel artists still use it, due to its simplicity. It has almost everything you need for pixelling anyway. But if you want something advance for ONLY pixelling, my recommendation is GraphicGale actually. GIMP is more of a free, down-graded version of Photoshop anyway, with lots of features pixel artist wouldn't need.
But that aside ...
I can see how your line is now a little bit better, as you no longer use 2pixel-wide lines. However, the form of the sprite itself is still funny to look at, especially the spike hair. If you don't know how to sprite that correctly, see some reference like the side view battler of Zack (from RM2k3) or this (
Holder's WIP on Eric's battler). It will give you some idea on how to do the spike hair.
Moreover, your coloring is a mess. As you color, it is essential that you specify a light source and stick to in all of your sprites. By having a light source, you will know which area should be highlighted and which should be darkened, and gradually color the whole thing. Here's an example:
Ignore how color is a mess in there, this is one of the sprites I did when I first started out pixelling
however you get the idea of how it is shaded. You can tell the highlight, and the shade will define the form of the hair, and such. You can study more the RTP and see how it's colored, too.
My suggestion stands still, read as many tutorials and possible and start with SIMPLE object. The link MA gave you is quite nice, Despain is a great teacher, even though it's not a complete tut, as there are many aspects of pixelling uncovered, but you will get the idea of the general spriting from him.