AI dont like to use troops for invasion, thats the reason valley of death achievement isnt very common, however in 1hour+preparation gametime you could farm the required amounts.

-play regularly against AI, choise hissho as opponent
-play as custom faction with fearless warriors 2 trait to have more planet defense
-set up supermacy victory so game dont end
-keep starting on tiny map until your starting system has 5-6 planets
-start to play, upgrade your planets to terran/jungle/ocean, develop all technologies
-build up all defensive stuctures on your home system and on 1 or 2 more system, keep in mind that more planets in the system means more population so more defense
-once done in a ceasefire against hissho give them all your systems except home and that other 1-2
-scrap your fleets
-save at the start of the next war
-lose your systems:-)
-RELOAD
-lose again
-repeat until you got the 200 kills acheivements

Each systemlose should grant you 3-5 troop kills.