It seems this achievement is broken or is it only available in campaign?
I checked the list of achievements on my friend's steam account. I am not sure that GoG has the very same achievements as steam but here is what I have already tried to do:
I tried different combinations:
Killed UrbanMech with UrbanMech's DFA;
Killed Firestarter -//-;
Killed UrbanMech with Firestarter's DFA.
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Other achievements which involve UrbanMech worked just fine.