
There is a yellow bar that fills up as the mob is 'casting'. If the yellow bar fills up while you are still standing in the ground marker, regardless of the mobs animation, you will get hit.
Some mobs cast faster than others. Also, I am sure some of the timings might be off and need tweaking.
Some mobs cast faster than others. Also, I am sure some of the timings might be off and need tweaking.