Change to resolve difficulty caused by new patch (cpu walk speed + Mercs).

This should be changed depending on difficulty rather than the high difficulty between the old and new versions.

I see it being tiered into the 4 difficulty levels as so:

Easy: The old walk speeds, allowing slower units to be picked off (Pre patch)
Normal: Similar to the above but less gap between speeds.
Hard: Similar to Below but with a slight difference in walking speeds.
Nightmare: The new walk speeds, solid wall of enemy (Post Patch)

These could carry over to Co-op based on combined AP tiering.

I think it would help keep easy and nightmare as per their names.

Additionally CPU Mercs could have their strengths reduced slightly as the difficulty drops from nightmare to easy.