You need something with magma, like magma bombs or a
magma volcano cannon. Add Lignite bins to the former to add to the carnage.
Oh, wow... I just had an awesome idea.

It looks like this:
______
█¢<% █
█ %>X█
█≈██¢█
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The 'X' is where you put the Bin (by any means necessary); on top of a one-tile retracting bridge or magma-safe hatch. Then you power the pumps, the magma should cycle through once (getting stuck by the hatch at the top-left) and the lignite should be burning. Then you open the hatch, dropping the Bin on whoever's unfortunate enough to be directly below. With some modification, this could even use a bridge-a-pult to fire the Bins instead of just dropping them.
The nicest thing about this design is that you only need a little bit of magma, and it doesn't get used up. The real ammunition, Steel Lignite Bins, is much easier to store and replace than magma is. Also, because the magma only briefly covers the ammunition, it might be possible to use raw lignite instead of a steel lignite bin, thus making it rediculously easy to reload.
PS: May I have permission to possibly, sometime in the not-so-near future, create a few much smaller and more spartan cutters and corvettes? Because my latest embark has a ton of vertical space, if somewhat lacking in the other two dimentions...