I was fighting against 4x4 square enemy(ogre). At the end of turn it was standing on one lava terrain and one acid terrain. It was supposed to die from the damage, but it only took damage from acid terrain.
It only takes damage from one terrain tile as per the rules, it probably counted the terrain closest to the bottom corner first, if I recall my rules correctly. http://www.cardhunter.com/how-to-play-card-hunter/#zeeatt
To add to Pengw1n's answer: In the earlier beta the big enemies did take damage from multiple terrain effects. That made the 'boss' fights too easy as you could easily do 40 damage per turn to a single enemy with just a Wall of Fire + Hot Spot. So the devs changed it to work like it does now.
