The terradons get there normal autohits against any unit (even a lone character) in a building. This was a slight change from 7th ed IIRC where autohits against buildings were treated like template attacks. Now, template weapons inflict d6 autohits at their normal strength if they touch the building with stonethrowers getting the higher strength if the centre hole is touching (BRB p. 127). Here are a couple other key issues about shooting at a unit in a building:
1. cannons cause d6 wounds for each hit inflicted so don't put monstrous infantry in a building!!
2. stonethrowers MIGHT do d6 wounds for every hit inflicted if the hole is over the builiding and the higher strength is being used. The wording on page 115 leaves room to wiggle where all high strength hits cause d6 wounds or only on one "model", as is the case when a stonethrower hits a steggie & crew. While I favour the one model ruling, I am biased since I am likely to be on the receiving end!!
3. Flaming attacks get to re-roll failed to wounds. This makes sallies totally rock at units that are bunkered and I have all but wiped out hordes of ~40 bunkered high elves/humans/skaven in two turns of shooting with multiple sallie packs.