Battle for Wesnoth

the main thing is not to let your upgraded units die

It's kind of hard to do, considering that the opponent is constantly targeting them.
I've only completed half of the first campaign, and I'm already playing with a save file that's been loaded and reloaded ten thousand times, and I'm undoing moves a hundred times because 40% chance happens four times in a row, while 80% or whatever it is, never happens.