there are two ways of doing so.
1st
add an 'critical hit' or 'bash' skill to the unit, with no effect except to reduce the damage to 0 if it succeds.
that way you can directly control how bad it is at aiming - but you have a whoel bunch of 0s showing up above the unit.
2nd
give every unit a 100% evade.
give the unit a 'critical hit' or 'bash'. Enable the 'allways hit' for it. Edit it so it does have no effect (100% damage, no bonus). Now the chance for it to happen is the chance for the unit to hit - and the damage it does is showed in red above its head while 'missed' is shown whenever it misses...
pick one - there might be more ways (like dummy-casting the spell that couses a unit to miss on it, or stuff like that) but out of the experience i got in mapping i would say that these two are the best once and the second one is the nicier one, even though it is more work to be done (and individual evades are not possible...)
whatever - i hope this helpes you thinking about the problem ^^
