Into the Breach

https://subsetgames.com/itb.html

Hero mechs save tiny people on an 8×8 grid, by using a range of incrementally unlocked strategic abilities. Interestingly, the AI heralds its next move so you can plan accordingly.

Genre: Strategy/puzzle.
Platforms: PC.
Difficulty: Different settings available, but this can get very hard as complex, interlocking elements are added.
Age-related suggestions: For older children and above who are good with seeing what might happen next and can work around danger.

Consider consulting detailed reviews before purchase.