You would first need to model the enemies, animate them, and then load them into Blitz3D. For now you can just use something like a cube for an enemy until you can model and animate some baddies :) You can use a Finite State Machine for the enemy AI. This is basically a collection of states such as Search, Run, Attack etc. When a certain condition is met, such as Health < 20 you would switch to the state 'Run'. There is an awesome tutorial on FSM's located somewhere in BlitzCoder. Do a google search for "Blitz3d finite state machines" and your bound to come up with something!