Unless they come equipped with cameras around their perimeters, anyone could sneak up to one and plant a couple bricks of semtex with remote detonators in strategic places and that safety gets real sketchy
I would not recommend trying that. Instead just dig a ditch across where they'd have to drive this thing. And keep the area around the ditch muddy. And suddenly it's a stationary gun emplacement.
Since i do not and cannot source semtex, i couldnt tell how long it would take to set it up. I can only speculate using the basic mechanical knowledge ive gained from 40+ years in the workforce and compare those timings with the physics required to rig up such things. Tl;dr: Just guessing