The ease with which such a drone can be shot down depends entirely upon its ability to evade a missile launch.
If the SLAM drone was a dumb weapon system, flying a pre-determined course, then shooting it down would simply be a matter of getting an anti-air missile in front of it, whether ground-launched or air-launched. However, given that this was the mission-profile for the conventionally powered Lightning Bug drones (in particular the Ryan Model 147S), a sufficiently low altitude would make it fairly difficult to shoot down.
However, if the OP's proposed SLAM drone had threat detection equipment that allowed it to observe hostile missile launches against it, then shooting it down would be an entirely more difficult matter. The fact that it is unmanned means that the limiting factor in its manoeuvrability is not the combination of the necessity to carry a human pilot, who also has a g-limitation of around 9g for a properly trained and equipped combat pilot, but the span-loading of its lifting surfaces and the strength of its airframe. A flying-wing or blended-wing-body shaped SLAM drone with low spanloading and even mass distribution could be very manoeuvrable indeed, and could conceivably out-turn a missile.
Modern doctrine for fighter aircraft evading an anti-aircraft missile is to attempt to turn behind the missile, effectively turning toward it. Why, when the missile is faster and more manoeuvrable? Why not turn away and open the range? The reason for this lies in how anti-aircraft missiles work. Observe a real (not Hollywood) A-A or S-A missile launch. You'll see that its typically solid-fuel rocket motor fires for a time and then burns out, usually well before reaching its target. Most missiles effectively coast most of the way to their target, relying upon their low static drag in order to retain kinetic energy. Most missiles cannot 'choose' to conserve fuel in order to prolong their manoeuvring time, their engines are like a firework that simply burns and produces thrust until all fuel is consumed.
The effect of turning behind the missile is to force it to manoeuvre as hard as possible. It is an aerodynamic law that lift produces drag (induced drag), so while the missile's fins are turning it, the act of turning - and producing lift - produces drag which slows it down, and the harder it has to turn, the more induced drag is produced, and the faster it loses its kinetic energy. A fighter aircraft cannot win in a turning race against a powered missile, but against an unpowered, ballistic missile? It starts off faster and is more manoeuvrable, but the fighter can power through its turns, while the missile's speed bleeds off, and with just a bit of luck, the missile will run out of energy before it gets to within attack range of its target, its speed will drop below that of its target's, and it will fall behind and eventually fall to earth.
So, if the OP's proposed SLAM drone can manoeuvre against incoming anti-air missiles, it would most likely need to be forked between at least two incoming missiles in order to have any chance at all of being shot down by any of them. It would likely be able to out-manoeuvre any single AAM or SAM launches, and would require having to choose which missile to turn away from and which to turn toward in order to be put in a position where one of the incoming missiles could retain enough energy to reach it. That also discounts the possibility where it could simply fly straight upwards under power, where the missiles pursuing it are entirely at the mercy of gravity, until they are all behind it, then turn hard to pitch downwards over the top of them, once they are all approaching from similar vectors.
So, a purpose-built drone capable of manoeuvring against anti-aircraft missiles would be an incredibly difficult target. Far better to simply fill the air in front of it with steel from dumb rapid-fire AAA and pray for a hit.