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Do you have all of the damaging set up correctly? Looks like after a certain swing is when the health resets, may want to check to make sure you’re not setting the health on that swing back to full. Also make sure that you are not accidentally resetting the health on the Boss’s blueprint.
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Yes that’s absolutely what it looks like and that’s what I thought first, too. But I have checked everything several times, but I’m not reseting the boss health at any time or at any attack. The only way the player affects the boss health, like in the tutorials, is by decreasing it through the min and max dmg variables which decreases the health of the boss, but not setting it up max again and the only time I setup the max health of the boss is at the beginning, when he gets initialized.
EDIT: I now know what the problem was. It is so stupid that I’m almost ashamed to admit it :o
So I setup the boss when the player overlaps the trigger box, right? …and his health, too… and stupid as I am I only set the triggerbox slightly above the ground so everytime the player uses an attack with jump force or where he comes out of the triggerbox and lands in it again the health of the boss gets reset to the condition of the beginning…to full hp 
and that was my problem
EDIT2: I recognised that this also occurs when the player leaves the triggerbox to the side and enters it again. To prevent that to happen I simply put a DoOnce after the initialisation so the player can brightly move in and out of the triggerbox without causing any problems