Perhaps hideously inaccurate, so that they land all over the map... like a Tremor turned up to 11. With mini-nukes.
Or perhaps an orbital bombardment - a spaceship lumbers across the map in low orbit (i.e. a very large, very high aircraft) and blasts things as it passes over. Give it a horrible turn rate so that it flies off the map and then comes back for another pass after several minutes, and a weapon that faces directly down and can only fire in a 1-degree arc so it can only fire at things almost directly below it.
Maybe introduce ground-to orbit weapons?
Sorry, I just had this idea in my head for a while and this seems a good time to mention it.