Grimrock

In sewers, where is the Ratling BOSS i did this. I went to the room the door closed behind me. I found a lot of ratlings and their BOSS. The health bar with the ratling BOSS showed. I paniced picked up the bronze key and ran to the door put the key in a lock and went back to sewers. After that i could kill ratlings that came out of the room easily. BUT the health bar of the ralting BOSS disapeard so i didnt know if i am still killing the boss or not. I tried for some time to do the damage to the boss but he didnt die - he has a lot of health. So i loaded saved game and killed everything in the room. So what i think is a bug that the health bar of the BOSS disapears when you go out of the room. It should show again when the BOSS comes closer to you.

The health bar displayed for this encounter represents all ratlings inside the lair, not just the big guy. My guess about the disappearance of the bar is that you left the combat zone of the boss fight.

Yeah that is true. But what i want to say is, that when i reenter it should show again to see how many health is left for the boss. But it doesnt really mind. What i was afraid of was, that when the health bar disapeard that i end the boss fight in some way. But i think not. Because i saw somebodys post on youtube where he showed the same tactics as i mentioned.

Aye and a very simple approach too.
Have a bunch of frost bombs ready (3-6), lure the big guy towards you i.e. right at the door, freeze him as soon as he shows up and them unleash hell upon him: melee, ranged,spells, fire/lighting/poison bombs, anything you got. When he unfreezes just toss in another frost bomb and repeat until he's down. Grab his key, open the door and then deal with the rest of the bunch.

After assembling the meteor hammer and exitting the CM, I was ambushed by that annoying Leprechaum with a cohort of rattlings, including another boss just like the one found in the sewers. I made a fast sprint to the hub teleporter and keep going in and going out of it, killing the unfortunates rattling that were stalking around the teleporter. After clearing up the area a little of enemies, I took care of the leprechaum and I noticed the big rattling boss where nowhere to be found. It is as if he just vanished in thin air. Maybe he got caught at the teleporter, but I doubt it because I couldn't find his weapon anywhere either.