Emitter Assassin - Destroy 20 Emitters in a mission
You will do that while doing "Size matters"
Any nice map to do
Runner Nest Assassin - Destroy 10 Runner Nests in a mission
Spore Tower Assassin - Destroy 20 Spore Towers in a mission
Air Exclusion Tower Assassin - Destroy 8 Air Exclusion Towers in a mission
Maybe Cavemaniac did Leapfrog map in XL version for that achievement?
Maybe try a DMD map? Some of those have large numbers of enemies. And if you can't find one, then make one.
Would it work if you made your own map (custom, not DMD), nerfed it so that you can get it too easily, and walked away just like that?
Yes. You can do it that way.
But nicer way is to do achievements in game :)
Pretty much, it nice that every achivment can be earn the fair way, just the Thor one I have small trouble, as I don't really use them, and if I did, I build 1, so the 100 total, will take a bit, lol.
Just replay the last story map and build Thors after each earlier one is complete.
Correct me if I am wrong but I think you can quick-save right before a Thor is built, let it finish, quick-load, repeat and get it fast.