Vizzy Cruise Missile

I designed a cruise missile, but i want it to fly to its target autonomously using vizzy.
I solved the problem of maintaining a consistent altitude with a proportionnal navigation system( basically a PID without integral and derivative because i can't make them)
Now i need a way for the missile to lock heading on the target only on the x and z axes, else it'll slam on the ground, ignoring Earth's curvature(if i just lock heading on target).
I'm thinking of a way to implement a way of inertial navigation by getting the heading of the target at start and keeping the information to adjust heading mid-flight
So yeah i need a way to fly to target. I accept any ways