In this arena a mouth appears out of the ground without warning. It seems to be the monster I should fight, but it seems to be impossible. Everytime it appears it takes a good amount of my health and I can`t do nothing against that!
Is there anybpdy who has won against this monster and can tell me how I can survive this arena?
I Like Hammerwatch very much, but this damn arena is too fecking unfair!!!!

I had the same problem with this boss and was close to rage-quitting, as I too thought that it was utterly unfair. However, I persisted with a strategy of circle-strafing the arena, trying to get shots off as I moved backwards. What I discovered was that the enemies wouldn't attack a lot of the time within the area that I was circling. When I went to certain spots, the enemy would appear and attack much more often.

I believe that there may be certain small areas, that act like a hidden pressure plate which when stepped upon with a shadow nearby will bring the enemy to the surface. I found one spot where it worked with such consistency that I was able to defeat the boss. It still took all of my four lives and I survived with 3hp and nearly died on the first beetle outside the arena!

The pressure plate (if it is such) that I found was to the left of the lower rock in the middle of the arena. I think they occur near to where the whirlpools form too (just to make it more frustrating). It may take some time to identify one of these spots, but once you do, it becomes a LOT easier. Just run over the spot strafing backwards and get shots off when the boss appears.

I certainly don't think the boss is as random as it at first appears, and can be beaten.

Normally I don't post on forums, but I hope this may help you or others who may likewise be frustrated by this fight.

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Edit: I played this game again to recreate this scenario with another character type and it didn't seem to work. I wish I'd been able to record it the first time to show exactly what had happened before. Although it was still possible to stay in the spawn point after dying for over 5 minutes without any mouths attacking, so I still believe there are areas where the mouths don't attack.

It's is difficult, one of the hardest bosses in the game to be honest. But all bosses in the desert campaign are much more difficult than the ones in the regular campaign.

Ok, so what you want to do is, that you wanna back as much as possible, make sure you remember where you have "safe spots" where the sand voretex' cant reach you. Stay away from the cliff sides, cause on several places there are flies that will attack you. When you back, hit the shark when he snaps at you, if there are a lot of them at the same place and around you, try and avoid attacking (stopping), cause then some of the other sharks might bite you.

This is probably too late for anyone already at the boss, but level up move speed as much as possible before, depending on your skill, level up hit points instead of damage too.

I played through as a Warlock on Medium and used a Keyboard+Mouse set up, which may or may not impact how this is received.

It seems to me that the best thing to do is to wait for one of the hidden mouths to stop and, when they do, step away and then into them and attack as many times as can safely be managed. I generally managed 2 shots but sometimes 3 before having to step away. It will take some practise to figure out the timing, viz. just when any given mouth will stop and reveal itself. The movement of the mouths seems to be completely random and this makes avoiding damage and the vortexs all the more difficult, however the other option was to spend far, far too much time attacking at range. Thus, it seems to me that the best option would be to increase 'basic' damage and become practised with moving in and out of attack range.