Each hit increases the cost to enter any hex that isn't water or swamp by +2.
A hex affected in this manner requires a piloting skill roll to leave that hex.
Each hit increase the piloting skill target number by +1.
Each hit decreases the targeting modifier against any unit ending their turn in that hex by -1.
(this target modifier doesn't apply if the target becomes immobile)
These effects stack 4 times.
It takes 2 turns of the hex not being hit to remove 1 stack.
I think having a hex that costs +8 movement points to enter, with a +4 PSR, and give a -4 to hit a bit nasty.
A Plasma weapon wouldn't really do this (or just this depending on the plasma type). A low temp plasma would have to have MASSIVE energy and amounts of generated plasma to do this. And if you did fire enough of it, you would have large secondary electrical effects also. (think of running about 2 or 3 Angel ECMs) A high temp plasma to do this would just turn it into molten rock, along with electrical effects, though not as severe.
Now a way to do this could be some sort of magnetic pulse device. (Not talking about an EMP, just a high gauss magnetic field. Though this would also have secondary ECM effects. None of these would cause a unit inside the area of effect to be easier to hit (other than maybe from just standing still). In fact, it might cause them to be HARDER to hit with anything other than laser and flamer weapons. ACs, missiles, MGs, and PPCs would be interfered with enough to decease their accuracy in such a high rated gauss field.
I would suggest 20 tons, 18 crits, and an ammo load of 3. Very high, but with the added effect of an ECM system understandable. Plus while very powerful, this limits the usage of it, thus helping to preserve game balance.