Summary:
A guardian/elder guardian's beam attack can rapidly damage the player and often kill them instantly even when equipped with a full set of diamond armor.

Steps to Reproduce:

1. Equip a full set of diamond armor and drink a water breathing potion.
2. Go underwater in an ocean and spawn a guardian or elder guardian.
3. Switch to Survival mode.
4. Let the guardian attack you. You will rapidly take damage and die/almost die instantly.

Original Description:
I enter an oceanic monument, kill the first guardian, in thath a small guard attacked me and began to suffer instant damage and had the totem of the immortality and instantly killed me an revived, it is as if he attacked 10 times in one second