I was just discussing this in another thread.
Give your Land Units a Ground Attack mission, but don't list any specific target. Also, make sure that they can fire at targets of opportunity. You can also give them an "Optimistic" setting, instead of "Pessimistic." Finally, you can let them shoot FREE, instead of TIGHT.
If you do this, these guys will fire away at everything. In fact, you MAY have to explore some ways to tone them down a little bit.
Units will ALWAYS work better if they are assigned to a MISSION rather than simply given a plotted or stationary position.
Hopefully, this will help. If not, perhaps I didn't completely understand what you were talking about.