I had the same issue ages ago. This may help;
To allow legs to move naturally whilst swiping a sword;
Backup the project;
Select a different layer mask to isolate the animation on the legs and allow just the upper body to animate during the sword attack.
by doing the following;
On the animator controller layers section;
Locate and select the Full Body Layer cog. By default it's set to 'Full Body Mask'. Just need to change this to 'Upper Body Mask'.
Click on the cog wheel, replace mask layer locate 'Upper Body Mask'' and apply . Make sure the weight is set to 1.
The 'attack 1 Sword /Katana animation now moves fluidly right or left foot is applied when attacking and no freeze on left or right foot.
After this, you may notice the death animation is not playing correctly (player is sustained in a standing position) if so:
The death animation requires the 'full body layer' to work, however it's on the same layer & wont play with 'Upper Body Mask'.
A simple fix is to put the death animation sub-state on a separate new layer.
You can literally copy the death animation substate create and paste into new separate layer (say call it 'Death Layer') with the 'full body layer mask' applied.
Delete the previous death substate. Death animation will function correctly again.