Around 2 hours 18 minutes to beat. The first checkpoint is tough. Theres no good indications of how damaged stonehenge is and whats damaging it. What worked for me was trying to take out all the ground units and taking a break from attacking them whenever bombers were announced over the radio. You will get coordinates such as 150 or 180, the altitude coordinate doesnt matter. Just make sure you adjust your direction to 150 or 180 depending on what they say. Get to the bombers as quick as possible and take them out as they do the most damage. I believe the A-10's and the SU units are the bombers. Once they drop the bombs they will leave, so dont bother following them if you let stonehenge get bombed.
For the last two checkpoints, fight the drones for a little while and it will trigger the final checkpoint. For the final checkpoint focus on the two propellers labelled as "Target". Stay close to the boss and take some hits if needed, but take out those two propellers than flee the area and ignore the other propellers and enemies. All you have to do is take out the two propellers labelled target and then survive for the remaining 2 minutes or so.
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