Hi Berry!
Monsters jump only if they need to reach a different platform. If you happen to stand on a branch and hit them with a spell from there, they might jump up to reach you.
At the moment, monsters can make one reactive move, even if the initial hit kills them. If the monsters that you attack just happen to stand at a position where they could jump up to you, they will do that as their first reaction, even though they'll die immediately after. This should not pose a problem; I may make the items they have drop to the floor wherever they happen to die, but apart from that, monsters dying while airborne should not be an issue. ;P
Toby
