If you are looking for a specific gear piece, look at the codex to find the floor and difficulty level requirements and go to that floor plus 1. Sometimes the requirements are the floor stated and sometimes it's plus one (like the enslaver feat to spawn legendary enslaver) so play it safe and plus 1. You don't want to go much higher then the minimum requirement because if you do then you are adding other possible drop items into the possibilities range which reduces the chance of getting the item exact item you want.
The best thing for farming items that drop from powerful mobs is to use the hunter mythic or asension perk, and play maps that have a high item drop and luck roll. People say this only applies to the carto and final chest but I feel like they apply to epic mobs too. Hit shrines to pop the epic mobs, kill, open new map, repeat. You can get a ton of drops quickly and just work the law of averages. Having high luck, magic find, and nadroji set helps (especially nadroji as it specifically increases the chance to find rare legends).
Just like many things in life, it takes forever to get a piece that you want, but the minute you no longer need it they start falling out of the sky.