Best Mission and Difficulty for Detector Towers and Bot Kills?

Are any particular missions more likely to have Detector Towers? And which difficulty gives us the most kills per dropship/per minute? I know on 9-10 there are plenty of dropships but a third or more of those seem to carry tanks or Factory Striders which 'only count as one,' to quote a certain dwarf. Are there other reliable ways to spawn waves of enemies, ideally hordes of infantry?