The Battle for Wesnoth is one of those amazing hex turn based tactical games, and they really are not many. Back in the days when I found it, I thought there are no more hex games I haven't play. And then tadaa - Wesnoth. And this game is free, I really admire its developers.
The screenshot is from after the 10th mission in the largest campaign. Back in the days, the game was crushing there, but now it is probably fixed. What I remember most, is that I needed to have at least one Elvish Champion, not only rangers, cavalries and healers, my favorites.