Not the most useful exploit since it's ridiculously easy hardmode, but regardless figured to share.
The bonus encounter boss in Black Talon Hardmode, robot that comes from the doorway.
Whoever has aggro, if he runs behind the carriage in front of the boss, effectively going out of line of sight, he will keep trying to target him/her, not move and not evade either,
having nobody taking any dmg, until someone overaggroes anyway, if someone does, they can just run behind it instead.
And yes, again, not the most useful exploit, but maybe usable for some really bad groups who have utterly crap dps not to manage the enrage timer or something.