Just some random floating idea here:
This could apply for a mob that wields a melee weapon that seems fit to be thrown as well. My idea is that one of their attacks involves them throwing said weapon at the player, with it being dodgeable. Regardless of whether the player gets hit or not, the weapon will then land on the ground. Naturally, the mob will rush towards their weapon and pick it up. Buuuut, the player also has the choice to race the mob for the weapon. Though the mob's speed when rushing towards the weapon could be faster than a player running, the player has distance on their side, so if they were to not hesitate, they will surely get the weapon. Once the player has acquired the weapon, they can then attack the mob for heavier damage or as the only method of damaging it. The mob will then snatch the weapon out of the player's inventory, and the whole sequence can repeat at anytime during the battle.
And to extend on the idea of "seeing exclusively through sonar" thing, I was thinking that this could provide the mob with some advantages and disadvantages. An advantage would be that they...I dunno, maybe sense the player farther away and through walls? Disadvantages could include having trouble telling where the player is when it is being surrounded by multitudes of sound around it, such as animals, mobs, flowing water, etc. And if you feel like it, you could also add an optional craftable item to help against this mob.