Here's a screenshot from a recent match in which I used lava and laser beacons against the yellow dragon: As you can see in the screenshot, the beacons are adjacent to the dragon but they did not fire. My guess is that it has something to do with the fact that large unit only get affected by one terrain attachment if they are standing on top of them. But laser beacons are different to other kinds of terrain attachments in that characters don't have to be standing on top of them - so I expected them to fire at the dragon despite the lava terrain.
Probably bug as discussed here: http://forums.cardhunter.com/threads/dropping-dual-beacon-near-4-size-enemy.7548/
