The biggest problem I have with this is that all of the action happens at the same time. There's no overlapping motion. Since everything starts and stops at the same time, the characters have a very stiff, robotic feel to them.
You could also stand to push the poses farther. Exaggerate a bit; that's one of the greatest strengths of animation. The thrusts lack force because it's unclear which part of each troll is leading its motion. that's due in part to the first issue I mentioned, but you can improve it further by concentrating the force into an obvious vector.
Basically, lead the thrusts with the pelvis and have the rest of each troll's body follow through. Also consider skewing their timing a bit, so that they aren't both perfectly in sync.
You could use more slow in/slow out around your keyframes.
Finally, your arcs need a lot of work. Things should be moving circularly or in figure 8s, not sharply back and forth.
Thanks for critique and advises!
newgrounds.com — Your #1 online entertainment & artist community! All your base are belong to us.