I think those mobs are a remnant of an unused design. How many camps and aetherial gates do not have any leves associated with them. Events and quests may call upon an obscure gate or location from time to time.

A level 60 mob is do-able in a full party.
A level 70+ mob is not. As you are correct, they deal 2000+ damage a hit and receive 1~7 damage.
Through great coordination, a server may be able to take down a single high level mob: Rainbow Basilisk, Guardian of the Grove, etc. even while doing 1~10 damage per hit and risking the retaliation of 9999 damage.

Some mobs, you react to sight ~ so running around their backsides may avoid alerting them.

Some mobs react to sound alone, so running and riding on chocoback alerts them, but walking does not trigger them even if they breath in your face.

If going for the achievement, I recommend running a zone every play session with a group of friends. Those that have the gate can act as decoys to buy you and others time. Taking turns as needed until everyone has the gate.