1- For the first mission, if you are on the top platform (level 2) can you attack the bad guys (range attack?)?
yes you can
2- Can you in this case use the special power of the hero (even if you are on the level 2 platform)?
yes you can (special power is for adjacency room (in the four directions))
3- In the rulebook it is written :
How is it technically possible? You destroy the highest pillar anyway?
Is it due to the ennemy who has the special fly hability?
It is not about the special fly ability. It is technically impossible with the setup from the rulesbook. Only if you create a bridge :
for example :
room - room on pillar (nothing below) - room
room - room - room
It was in my different setups.