Well, I managed to do it. From the 4th attempt that is... I was simply trying to make a point. People talk about medals being illogical, hard to get and so on. The trap medal can be both a boon and a curse. In this case, it was a curse. Since I'm a mage, I can't get the tactics skill to rearrange my troops and have to physically move them in order for the stronger troops to defend the weaker ones. If I move them, they go on a square next to a trap, begging for the emerald dragons to have some fun. If I don't I leave the weaker units unprotected, and the dragons simply go and have some fun with them.
It is a very irritating situation, but as usual, there is a way out of it. You just have to look for it