The last time this golem assumed that weird posture, it discharged a disruptive shockwave that knocked everyone back, and then sprung into an unavoidable speeding rocket. Jack was oblivious of its move the first time, but he was not going to be caught off guard for the second time.
The second the golem squatted down, Jack channeled his concentration to form a spell rune, while warning the others, "Move back!"
As predicted, the disruptive shockwave soon appeared. It knocked Jack two steps back as he was especially close to the monster. His concentration was broken by the shockwave, but his gloves helped to maintain the spell rune from dissolving. He quickly resumed his spell channeling.