To avoid any frustrations, just cast detect invisibility in the cave near the chest. You are better off walking toward the north side of the cave to take out the umber hulks first, since they will attack when you click on the "chest". Once you casted detect invisbility, you will notice a invisible stalker lookalike standing in front of the chest. Reminder, mimic cannot move, hence start shooting arrows and casting spells.....although this is not suppose to be a tough fight.
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Long live H22A1!!!!!