These would be the ones from Apoc I think. Lobstermen can't stand those something missiles (forgot name) after all. The rest can't either. In air combat though... EDIT: I just realized you meant a single alien... Micronoids?
My way for dealing with entropy launchers is simple. When an agent gets hit, he best thing to do is drop all your equipment and armor, and run to the nearest exit. Then I try killing the launcher...does'nt always work...
It does sound like it stopped in the middle of a track or something not that I understand these things much (so I'm waiting for someone who does to respond).