But if they are a building I'm not sure anything is actually "Good" for killing them w/o pasting the building in the process.
What are the mission goals, just kill them? Are there any other considerations?
Sometimes leveling a town or even a building may not be feasible.
If you don't have a warship or nukes handy, those options are off the table. If you are running on a slim budget a battery of Long Toms might not be available and if you are low-tech, stuff like infernos are also crossed off the list of options.
If time is no issue, surround them and starve them out, cut off water and power and they will eventually surrender once their supplies run out...
If I allow them to surrender.
Any civilians in the area are given time to evacuate, anybody left is declared a combatant, hostage, or human shield, and I don't negotiate with terrorists after I allow them to surrender.
I have no qualms about using chemical weapons on lower-tech troops who lack adequate protection or training -They shouldn't have picked a fight with adults.