Force Punch is Thekal's most noticeable and devastating effect. The raid will be aware of when what is coming when Thekal leaps into the air. Upon landing, he will hit everyone around him for approximately 1,000 damage and knock people back.
The range on this AoE is large enough that healers at max range are still affected by the knockback. As a result, in order to counter this attack, people should keep their backs to a wall to mitigate the time lost when knocked back. Hitting a wall will allow you to land and get back up as quickly as possible. There are certain areas in Thekal's area that will almost cancel the knockback effect.
Thekal has been known to do this twice in a row on occasion. The raid should always be prepared for this by staying above 1.8K health whenever possible.
When he does Force Punch you can jump as soon as you see him start the animation. This will not only keep you in one place, you will not take damage.