With the release of Fontaine in the Genshin Impact 4.0 update, new types of commissions have also become available that you can complete to get free Primogems. One of these commissions is called "Don't Get Crabby" and will ask you to "Destroy all of the Bullet Barnacles." There will be a total of 3 Barnacles that you need to destroy, and here's how exactly you can do it as if they will give you heavy damage constantly.
Genshin Impact: Destroy All Of The Bullet Barnacles
The commission will take place in Salacia Plain at the underwater location marked on the map above, and to complete the commission, you will have to first take the energy of Fontemer Aberrants Crab and go near one of the Bullet Barnacles.
Use your elemental skill (E) to attack the Barnacle and destroy it. Upon destroying the first Bullet Barnacle, you will unlock An Eye for an Eye achievement. After that, repeat the same thing with the other two Bullet Barnacles in order to complete the commission and get 10 Primogems.