The easiest way to do it is with a crossbow and a rope.
Anchor the rope on one side, fire the crossbow with a weighted bolt and rope attached, tie the slackline to the rope and pull it across.
There are anchors for rock climbing that can be either drilled into rock or put into cracks and crevices that can support the weight. They can also use sturdy things like trees and boulders as anchors.
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