My favored unit for the Stone Sword has been the Harbinger for the good MR. So long as the battles dont last too long you can get a lot of mileage out of a cheap chasis before it kills itself with the sword.
No unit can safely use the stone sword. But I usually buy it and keep it in the lab so it cannot be used against me. Same with the ark, unquenched sword and a few other items.
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"War is an art and as such is not susceptible of explanation by fixed formula."
- General George Patton Jr.
Yes, thats why i said "... before it kills itself with the sword". But its silly to build the sword and not use it. Its quite powerful and if you manage it properly it can be a very good weapon.