To get a non-guided missle you need to decouple the target from a unit to the ground under the unit. You launch the missle targeting the ground so that when it impacts it will run the effect on the ground. Instead of a normal damage effect which targets a unit you perform some kind of AoE search (eg what EMP or psyonic storm uses) to deal splash damage at the target point.
I do not know how to make missles which explode when near an enemy unit (launched toward target but explode on contact).