It's almost impossible to get this prior to liberating Ilapa due to the lightning storm surrounding the mountain. Almost impossible.. in that you could get lucky and land in the right spot before you get annihilated. It's better to wait until you have liberated Ilapa by completing the storyline.
The highest point in Solis can be found at the following location:
If you're having difficulty working out exactly which point it is, you should go to the top of the highest tower which is this one:
Once on top of there, you then need to make your way across to the exact point I'm aiming at (it doesn't look right cos of the angle but it IS the highest point) and wander around a bit on top of the rock until the achievement pops: