One thing I would recommend is to REALLY focus on those mannequin skills. Every artist does mannequins a little differently (some artists have more than one mannequin they use) so don't get hung up on any one system, just pick something simple to start with (Proko's robo-bean + cylinder limbs is a great start) and carefully invent poses, focusing on rock solid clean lines and perspective (sloppy drawing will really hold your mannequins back). Over time it becomes easier to visualize things, and you'll learn lots of bone-stock generic poses that work well which you can modify (the "Masters of Anatomy" books are pretty good references for go-to poses, but make sure you're constructing a mannequin instead of just copying his roughs). Secondly, to echo nnnnnnnadie's comment - get into those poses. When you imagine one, if you get into the pose, you'll quickly realize if it's awkward or impossible. Once you can find a pose that actually feels natural you can snap a photo and draw a mannequin. Eventually you'll want to work from gesture > mannequin > drawing, or even straight to a pose Kim Jung Gi or Stan Lee style. If you struggle with getting your mannequins in perspective, just work on simple solids like cubes and cylinders until you're fairly comfortable. Also... if you get stuck, fire up Daz or Magic Poser or something and just pose away. It's best for learning if you can work out the mannequin yourself, but if something keeps coming out wonky - no rules, only tools. Knock it out and try to get it from imagination next time.

Fantasy poses request an ideation of kinestesics of the body throught the use of tools/weapon from (usually medieval era), So, the first thing you have to do is to ideate a character and contextualize. for example a barbarian warrior, how do you create a natural pose for him/her? you contextualize your character, when do you want to capture the pose? after battle? how would a person who is swinging a sword all day looks like? his/her face has a fierce look? then search for reference, look at videos of how the heavy lifters body rest after a lifting or something like that. Bottom of the line i suggest you use your own body and an "avatar" (like a boomstick) for the weapon and start feeling the agressivity and start mashing pillows or whatever makes you feel a warrior, get all on video (important) and try to get a pose out of it, funny yes, but its a way, another way is to observe 1000 poses from comic books, there are a lot there, look how jim lee hush has a lot of it. Draw constructions of the poses from reference first, and after you have drawn 100, imagining them becomes easier.