Man I feel dumb, but I never destroyed that building, only moved my titans onto the hex. Thanks for the tip!
In the beginning of the campaign, I was attempting to use heavy and assault titans for each mission, but that became almost impossible to succeed, so for the last several missions I have gone with the pre-mades Imp (70), Unicorn (40), Thor (70), and Vulcanoid (70). The lighter titans make the difficulty manageable, though I would still classify most of the missions as highly difficult (especially the one in the city where you rescue the general). I'm not complaining - the difficulty is what makes the fun to me, but you better save your game often .