You can get Inspire ____ mercenaries achievements by using IMSP, which effectively decuples the mercenary camp you are using IMSP on and then inspires the camp. Thus, the first level that Inspire 10B mercenaries can be achieved is Breaking Green, specifically by conquering Hasting (About 1.1B mercenaries there according to the level stats page by PkmX and FiveSmith I used to write this), then using IMSP on its mercenary camp. This acquires about 11B armies, which is greater than 10,000,000,000.

I am not a crab.

Edited 8/11/2023 19:50:44