I actually used 3 types of putty in this one, thought one of them was really just to block the armature. All three have different properties, and thats why I used them that way.
First, I used Milliput to block the wire armature. Milliput is a 2 part epoxy putty that turns rock hard when its fully cured so its perfect for that. Green Stuff would be just as good for this, but Milliput is cheaper and I have a box of this black one that I want to get rid (you cant really see what you are doing with black putty). Thats mostly why I used it.
Then, I used Green Stuff to cover the armature with just a thin coat. Green Stuff is very sticky and the adhesion to the wire its pretty much perfect. Fimo would never stick to a raw wire and would just keep bouncing around, but it sticks perfectly to Green Stuff. It also helps to block the pose, as it cures hard enough for that. I also did the leaves and vines with it, later on.
Finally, the body was all done with FIMO. The main advantages is that I can find it pretty much everywhere cheap and it never sets, unless you cook it in a hoven. So, you can do stuff, remove, do again, etc, without wasting any material. Its a bit more like working with plasticene instead of chewing gum, if you get what I mean.
You are right though, its not as resistant as Green Stuff or Milliput, but it cures hard enough to make a display piece. If you plan to use this for gaming, it might be tricky for the thinner bits. I'd probably make a mould and cast it in resin if that was the purpose.
On a side note, I absolutelly love Milliput. Dries rock hard (utra.resistant), you can sand it (easy smooth surfaces), it can be dilluted with water (sticks great and you can use it for gap filling with a brush) and, this is the really nice bit, you can mix it with Green Stuff and get that elasticity too!