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There are a few key items to focus on for MF which require different strategies. Unique & Set items. Socket bases (runeword bases). Jewels. Charms. Gems. Runes.
I'll run through a quick summary of each and then detail my farming strategy and results for you to understand how I've acquired some of the best gear in the game using my cold MF sorc.
Unique & Set Items:
These can drop from any enemy or chest but each item has a specific item level (ilvl) assigned to it which can only drop from specific monster levels (mlvl) and certain difficulties and areas yield the best chances at items.
There are plenty of forums and sites out there detailing where to find the best probability for each specific item in the game so it's my recommendation to google for that. Diablowiki, d2jsp, rankedboost, and others are my go-to.
If you're looking for a top tier item but it has a lower equipment base or ilvl to it, then your odds are usually higher to farm lower level areas. For instance, I've had 3 occulus (occy) drops. 1 from nightmare Diablo, and 2 from nightmare Baal. My only shako dropped in Hell Act 1 from a random fallen shaman.
Socket Bases:
These are items that drop as "white" or normal rarity items. Sometimes plain white, sometimes ethereal (eth), sometimes already socketed, and sometimes eth and socketed. These are used to create runewords. Everyone will tell you to farm cows and the less magic find (MF) you have the better. This is because 1) cows are dense and drop a ton of items and 2) less MF increases your chances of normal rarity items.
The other things that I've gotten the most socket bases from are equipment racks (armor and weapon racks).
Jewels/Charms/Gems:
I bucket these 3 together because anything in the game can drop them and it's random. Ensure you stay well stocked on chipped and flawless gems. Flawless you convert 3 into a perfect gem (pgem) which are used for crafting, or you can trade 40 pgems (an entire inventory full) for a near high rune.
The better stats on jewels and charms come from hell and the best spots to get these are in mob dense areas like cows and chaos sanctuary. Most of my skillers (grand charms with +skills) have dropped from hell chaos sanctuary.
Additionally, if you're looking for Gheed's then I have had 8 of these drop; 4 of them were from hell Andariel.
Note there are very specific stats to look for on these so it's best to look at my trades guide to understand. Most notably check game lobbies, discord, and forums to see what people are looking to buy. If you land the right stats you can get very nice trade value out of jewels and charms.
Runes:
Everyone wants to know "where can I find a Ber or Jah" and the answer is not what they want to hear. The honest truth is runes have a chance to drop from literally everything in the game but your highest chances are Super chests (especially lower kurast in a players 8 game), mob dense areas (like cows, chaos sanctuary, etc), and ghosts (because of their limited drop table). The countess also guarantees at least 1 rune drop and Hell countess can drop up to Ist.
For starting point I'll tell you I've seen Zod, Cham, and Ber all drop in cows. Another Ber dropped in the forgotten tower on my way to Countess. I've also gotten a couple Ohm & Vex from arcane sanctuary. For this reason I usually try to clear areas as much as possible instead of teleporting past enemies to get straight to bosses.
MF Gear:
My MF build is a frozen orb sorceress with firewall as my off spec skill for cold immunes.
I run the A2 Might aura merc with an eth thresher insight (any insight will do but eth thresher is best in slot) so I can teleport forever. Note, you'll want high defense/resists on chest (I run upped duriel's) and life steal on helm (I run crown of thieves). You can also throw an increased attack speed/resists jewel in your helm and armor if you want.
I'm wearing 3 piece Tal Rasha's set with occy and shako instead of full Tal Rasha's. (Note: if you don't have a Tal's belt you can run a goldwrap). You can choose to socket your helm and chest and put an ist or Ptopaz in them to boost your MF more easily.
Wartravs for boots are best in slot (note: you can run yellow demonhide boots with similar stats until you find travs).
Upgraded chance guard gloves are best in slot for MF gear.
Offhand I use a Rhyme heater shield (Most will tell you to run spirit monarch in the offhand but a rhyme heater shield costs you less strength and gives you better blocking & MF, but sacrifices defense, cast rate, and skills).
For rings I ran dual Nagelrings for a while, but have since swapped out one for a ring that gives me better resistances and cast rate to max those stats out.
Note that MF % has a falling off point around 350-400% as the return on investment gets a lot lower the higher you get. I've felt like ~400% feels like the sweet spot for clear speed versus best chance at drops.
MF route depends on what you're farming for, what your build is, and what you feel comfortable farming, but I'll list my full route that I run multiple times per day on my cold sorc. Specific note to not teleport your way past all monsters to a specific boss or chest. The more monsters you kill the higher your chances of loot so if you see a large pack of non-immune mobs or a champion/unique then stop to kill.
Act 1
- Mausoleum: Take WP to Cold Plains, teleport along path into the burial grounds, kill blood raven, enter Mausoleum (always upper right graveyard). Kill the 4-5 champion/elite monsters and loot the 1-2 super chests.
- My best drops: mid-level runes, items worth a few pgems, gems, jewels, socket bases
- Secret Cow Level: Take WP to Stony Field, find cairn stones, kill rakanishu, enter tristram, loot wirt's leg in far left corner, use tome of town portal and wirt's leg in cube to spawn cows.
- My best drops: zod, cham, ber, socket bases, jewels, charms, gems
- Countess: Take WP to Black Marsh, find the forgotten tower, teleport your way to level 5 killing all champion/uniques you find along the way. Countess drops runes up to Ist and a chance at a Key for Ubers.
- My best drops: Ist, Mal, Tkeys
- Andariel: Take WP to catacombs level 2, teleport to levels 3 then 4 (kill anything you want along the way but avoid ghoul lords).
- My best drops: Gheed's, uniques worth pgems
Note: The pit is another mlvl 85 area but I skip on sorc due to too many cold immunes. You can choose to tele to the super chest and if you want to it's closest to black marsh or outer cloister waypoints then go to Tamoe Highlands.
Act 2
- Ancient Tunnels: Take WP to Lost City. Find purple dot on map that says trap door. Click to enter tunnels. Clear the 4-5 champion/unique monsters and open the super chest just like in mausoleum.
- My best drops: eth reaper's toll, gheed's
- Summoner: Take WP to Arcane Sanctuary. Go down each of the 4 paths. Note the 3 "wrong" paths each have super chests at the end with a chance to drop runes so do a full clear. Summoner also drops a Key for Ubers.
- My best drop: reaper's toll & Hkey
Note, Maggot Lair in Far Oasis also is mlvl 85 but I skip this as it's a pain to clear and has cold immunes.
Act 3
- Lower Kurast: Take WP to LK, teleport to a camp fire with stakes in a circle around it. Open the chests in the 2 buildings adjacent. Kill any champions/uniques and click on chests/racks along the way.
- My best drops: Mid runes, gems, jewels, charms, mid uniques/sets, socket bases
- Council Members in Travincal: You can choose to do this or not. They are tough enemies so sometimes I skip if I'm doing a faster run and don't feel like losing my merc.
- My best drops: Mid runes and mid uniques/sets
- Mephisto: Take WP to Durance of Hate Level 2. Teleport around outer border until you find level 3. Note council members in level 3 can also drop high runes but can be tough to fight solo so your choice to avoid or fight.
- My best drops: Mid uniques/sets
Note, there's 3 other mlvl 85 areas here (ruined fane in bazaar, disused reliquary & forgotten temple in upper kurast) but I skip these as they aren't fast to get to and contain cold immunes.
Act 4
- Chaos Sanctum: Take WP to River of Flame and teleport into sanctum (upper right of WP) and clear it. Press all seals and spawn Diablo.
- My best drops: skullder's, skillers, occy
I sometimes skip this Act if I'm doing fast runs as the cold immunes in here are tough to deal with and the merc AI doesn't target them well.
Act 5
I usually only do Pindle (from portal next to Anya) then save and exit at him to start my runs over.
My best drop: Tal rasha's guardianship chest
However, some folks also farm eldritch & shenk off the Frigid Highlands WP but both are cold immune. Additionally, Worldstone Keep and Baal are mlvl 85 if you're up for the challenge.
Good luck heroes!