There's a bug that let's you raise your Strength stat without much effort: First, the merc in question must carry as much heavy items as possible. "Hospitals" are the heaviest items in the game, so you can load up 6 Hospitals in your merc "large inventory slots" in addition to other items in the "small inventory slots". This should weigh your merc around 300% or much more, depending on the current STR of your merc. Second, go to the overland map screen. Here assign your merc to travel to any other sector. Confirm the travel orders (without accelerating the time sequence) and then cancel the orders. You will notice after 1 or 2 tries, your merc will have got their STR stat increased! This should help your merc raise stats to around 80. After that it takes several tries to raise the STR stat.