Where to do Assassin achievements?

[Mefi]

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?

[Randomgold]

Maybe try a DMD map?  Some of those have large numbers of enemies.  And if you can't find one, then make one. 

[4xC]

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?

[Mefi]

Yes. You can do it that way.
But nicer way is to do achievements in game :)

[Tjgalon]

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.

[4xC]

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.