The Dragon's breath does about 3 hearts of damage per second, and diamond armor, Notch Apples, and totems don't help much. The acid is also invisible. This, along with many other bugs that have already been reported make the End fight basically unplayable, especially if you are speed running.
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Thank you for your report!
We're tracking this issue in MCPE-40344 and MCPE-51214, so this ticket is being resolved and linked as a duplicate.
Those tickets have already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game, you can check the Fix Version/s field in those tickets to learn more.
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this is fixed in the nether update betas