"Varas" Enemy unit has an infinite sheild

This may or may not be a implied feature (if it is, why?)

But the Varas enemy has an infinite shield only allowing me to do 1-2 damage on a normal attack, and 8 on a rare critical hit (weak spot). For clarification, I was a level 12 Braver using a level 8 Bow and the unit was a level 10 Varas.

Additional Info: When shooting all arm projectile units revealing the weak spot, the Varas unit would still keep the shield and the weak spot would never trigger and do the additional damage.

If it was one of those enhanced enemies that have a red shield around them, you need to attack the big glowy thing sticking out from somewhere on their body and damage it until it breaks, and then you can attack them normally. There's even a cocoon you need to complete to progress that tells you about it.