Texturing organic things like creatures and characters is pretty different from texturing hard surface stuff (props and assets), atleast to me, that's why you have to approach organic and hard suface texturing differently. If you want a good texture for a monster or a character then it's firstly a must that you have a graphic tablet (Wacom Bamboo is pretty cheap and the best one for a start in my opinion, I use it aswell and I think it's great). Then there's few things you have to keep in mind and follow to be able to get the best end result of both the low poly model and the texture:
1. The sculpt. This is the most important thing for making a detailed and good model for a monster and even FOR the texture (I'll explain later why). Having atleast a basic knowledge of anatomy will help you ALOT in making a good base sculpt for your characters/monsters. It's the big shapes that matter first when sculpting (that's why you don't subdivide the damn mesh to the max from the start) and details comes last cause they wont help if you're main proportions and big shapes are bad. Then for the detail for skin, pores, wood or other stuff depending on what type of material your part of the sculpt is, using textures as alphas or stencils will bring alot of detail to the sculpt. Using textures like stones, dirt or bascially skin pores will work for the most if used the right way. Remember, its the big shapes that matter first, then defining the material of the sulpt (hard and soft edges) and then the biggest details. Small details comes last.
2. Polypainting (or 3D painting in Mudbox) is pretty much what most character artists use to make the main texture which is basically painting on your sculpt directly and not on the UV in PS (You're painting the verteces to be more specific). What I do is bascically give my sculpt's a base color (let's say the grunt base color is skin color so I just make the material color of the sculpt that color), then I take a normal brush (depending on the surface) and give a base color to smaller parts (main details) of the sculpt (a rope maybe, mouth, nose, bandage, whatever, think you get it). After I gave the base color to the parts of the sculpt I start using alphas which basically makes the brush use a texture that I imported (called stencils aswell which Mudbox uses). While using the alphas I choose different colors for the brush that fit and I start painting on the base color of the sculpt which makes the textured strokes of the brush blend with the base color and gives it nice detail. Remember to have your sculpt as high poly as possible for best polypaint result as it relies on the verteces (applies to sculpting the detail aswell). If you're sculpt is pretty detailed before you start polypainting it it will help and save a bit of time since you have most of the details already. This guy shows pretty well how to approach polypaintign characters:
http://www.youtube.com/watch?v=j0J3nvMvXGg
3. Baking the maps and cleaning up in PS. After you've done with you're high res sculpt and polypainted it you will have to retopologize it and make it low poly (newest Zbrush has a new feature though that retopologizes the sculpts pretty well in one click, it's not perfect, but it's great for what it does and saves alot of time). Then you'll have to unwrap the low poly mesh of course. Then comes the baking. I use Xnormal which can bake most types of the maps. Normal map, Ambient Ocllusion, Cavity Map and Vertex colors is what I bake. Normal map will show all your details from the high poly sculpt on your low poly sculpt (that's why it's important that you have good detailed sculpt). For the diffuse you use the other 3 maps. Vertex colors is the polypaint that you painted, then you place the ambient occlusion and cavity map on the the vertex color map and make the layers' type to multiply in PS. AO and Cavity Map will give you're texture deph and will bring out the details from the sculpt. Then you can also make a specular map if you want aswell using different techniques. When normal map, diffuse map (with AO and cavity applied) and specular map will come together on your low poly model it will looks good just like the grunt (pretty sure that's how about they made it). Now when baking maps you'll probably come across some problems and you're maps won't come up perfectly, but that's normal and there are different things you can do to make you're maps perfect (you'll have to find that out on your own, sorry).
This video is pretty good showing the main things on making a good character and explains things that I told pretty well so I advice to watch it if you want
http://www.youtube.com/watch?v=vbCl66NFTSs
In the end, there are alot of different ways to approach making textures for characters/monsters which are still a bit similar in a way. It just comes to practising, trying different techniques and finding what you think works best for you. This is just what I found that works best for me. I started by using Mudbox (which you can get for free from Autodesk Educational Website) and it worked great to learn the basics of sculpting and 3D painting and it was really easy to use so I recommend you start with it. This is one of the first monsters I made using it, It's shit to me now couse I improved alot ever since and Im using Zbrush now but I think it's ok for a first monster lol
http://jurasbatas.deviantart.com/art/Ski...-407983564
and here's the video of me 3D painting it if you're intrested to see my approach
http://www.youtube.com/watch?v=V7wsbS6fhrQ
Like I said, it's shit compared to my current skill and I even painted on the low poly mesh lol, so if you want to do similar approach do it on the high poly sculpt instead. Anyway I hope this helped, I know you're frusturation, I had it too but just don't give up, start learing a bit anatomy (it will help more than you think) and I also advice watching some of this guy's videos, he's really a master in design and you'll will learn alot and get inspiration just by watching, trust me.
http://www.youtube.com/channel/UCbdyjrrJAjDIACjCsjAGFAA
I might make a tutorial thread for everyone some time showing how to make monsters for Amnesia (I have made 3 monsters so far, animated them and made them work in Amnesia so yes, I know how to do it). But until then I hope this post helped you, good luck!