Mars: War Logs

Interestingly, the game does not actually have anything called "Difficult mode". It does however have a Hard mode, which is what this is really referring to. It also stacks with the Mutant achievement, so you're better off just playing through on Extreme to grab both achievements. See NIKKI ZERO COOL's excellent solution for that achievement for a lot of useful tips to help survive that incredibly unfair experience.

Good luck!