I like the sword, but your attack animation really isn't communicating the weight effectively.
In general, try looking at the position of only part of of his body while stepping through to see if they describe clean movement paths - currently his feet jiggle around meaninglessly, and the head position isn't much better..
This is most important with the sword - heavy objects cannot (without enormous force) make sudden changes of movement. Once they start moving or rotating one way, they're probably going to carry on like that.
If you want him to finish in that frame after swinging, he needs to be taking a step forwards during the swing - currently his right foot is sliding back a lot in between frames 5 and 6 (which also makes it confusing which leg is which, since they more or less swap position at that point)
I don't understand why he rotates the sword as he lifts it before the swing - it would make it require even more strength. And you'd get more air resistance, trying to start swinging it like that.
After thinking about it a bit, I reckon you'd want him to lower his body a bit at the start - this way he can start the sword rotating, and crouch down to start lifting its weight at the same time.
Maybe also move the frame before the impact point back a little, so the impact frame has the biggest motion (and have the most motion blur on the impact frame)
This stuff's hard to describe, so here's an
incredibly messy (

) edit to hopefully clarify what I mean:

It would also be good to add some sparks or something to the impact frame to emphasize the blade smashing into the floor.