Into the Breach
Into the Breach is a turn-based strategy game only on PC from developer Subset Games, the makers of FTL: Faster Than Light. Instead of controlling sections of a spaceship, the game gives you control over a squad of mechs against an alien threat on an eight by eight grid.
Like the previous game, Into the Breach can be a punishingly brutal game where every move must be scrutinized for maximum effect. One turn you could be fine with a full squad, then have them wiped out in the next turn because they were in the wrong positions. Every challenge is randomly generated, so it is a new fight every time even though you will be visiting the same places since wiping sends you back to the beginning.
It is great roguelike for $15 that can be played for 20 minutes or more at a time thanks to the variety of enemies, weapons, and tactics that can be used.