Can you show us how characters will oppose each other?
Something like a group or a line?
Or something else?
Also what software are you using to draw/animate?
Here's my thoughts on this.
You have a lot of restrictions.
But no worries.
You can still squeeze a cool animation out of something like this.
First I changed a few things about the sprite.
Modified his face a little to better match his center line and make him face more to the side.
Put his sword arm to the back.
Added a shield to the front since you said you will have them as optional.
It's a good idea to fully load your character so that you know what he will look like.
Then I broke him into the following layers.
Then I made a simple attack.
The head and armor are static drawings as specified, only their position changes.
I left the shield unchanged as well.
Ignore some of the messiness.
Didn't take the time to draw in the other arm.
Sword is quick and dirty.
etc.
Also with combat animations it's good to mockup interaction.
Judging by your restrictions, especially "has to be glued into place.", I assume you are going for something rpg battle style.