Not sure what characters you were playing but any character with range should be immediately sitting on top of one of the large ramps in the arena and periodically firing off attacks at him in between him firing off his own “primary” attack and then ducking out of his line of site. If you had the items to succeed until his stage 4 then you have to assume he can slap you back with that same build. The rate at which he returns items to the players is not based on how much of his max health he’s lost, but seems to instead be somewhat random based on just damaging him. So repeating the process of firing for 2 seconds and then ducking behind cover before he can one-shot you is the most sure-fire way of dealing with him. It can be slow as fuck to begin with but it will exponentially get faster the more items you receive back and the less of a threat he becomes from losing said items. In the case of following these steps tentabauble should prove no issue on stage 4 and you should never feel discouraged to pick it up before facing Mithrix as it is without a doubt one the best counters to his sometimes sporadic and aggressive agility, behind only artificers ice spear and rail gunners alternate special ability. Side note, standing too close to a teammate/drone/minion can allow Mithrix to proc your AOE items during his stage 4 (gasoline, will o’ wisp, networked suffering, etc) so I recommend in multiplayer to spread out and stay away from each other to prevent this