[outdated] Caustic Arrow Solo Map MFer (20/300+)

100 Atziri Kills:
Not recommended.

Sacrifice sets cost about 7c each so it'll run 700c or so to buy all the sets. Moreover, Mortal Hope fragments sell for about 30c with full mortal sets going for a bit over 1ex. The much lower price of mortal fragments this league is likely due to the availability Guardian/Shaper drops. (Mortal Hope prices are relevant because if they were more expensive, you'd be better able to offset the cost of this challenge by selling the Hopes you accumulate during your Atziri kills).

This challenge is not recommended because it's expensive, it will take a long time to complete, and whether you plan to purchase or farm fragments - it will take quite a bit of effort to get all the fragments you need.

I've been purchasing fragments using a Public Party and by looking for other Public Parties. This method for purchasing works best but still takes awhile.

The Apex of Sacrifice does counts as a "map" for purposes of possibly holding the essences you'll want to corrupt (Dread, Envy, Misery, Scorn); although you'll rarely see these essences, over 100 runs you should get a few essences to corrupt.

Edit - changed post to note that Dread/Envy/Misery/Scorn can appear in Apex.

60 Izaro Runs:
Tentatively recommended.

Offerings go for 4c each so it's only 240c to buy all the Offerings you'll need. If you have a build that can rapidly and easily do uber lab runs, this challenge is definitely attractive.

30 Eater of Souls (T15 Core Map Boss) Kills:
Tentatively recommended.

Core maps cost about 20c each so 30 maps will run about 600c (half that if you can handle twinned). On the other hand, if your Atlas completion bonus is high, you will get some high end map drops while completing this challenge so long as you chisel and roll pack size and quantity on your maps.

The downside of Core is that mob density is low. There won't be a lot of mobs to kill and so map drops will be lower than for other map layouts.

30 Cursed King (T15 Dark Forest Map Boss) Kills:
Highly recommended.

Dark Forest maps go for 18c each or lower. And the end boss is easily killed twinned for some builds (but might not be doable twinned for others). Assuming twinned kills, completion will cost 270c.

The reason that some builds find this encounter easy, is that there's a 4-second or so dead time when you first come within range of the bosses before they'll attack you. This allows you to carefully choreograph your opening attack - lay down traps, put down maximum mines, carefully place totems around bosses, drop a decoy totem behind the bosses, etc. They'll remain in the starting place once they begin attacking your for maybe 8 to 10 seconds. During that time frame, do maximum DPS - pop flasks, Vaal gems, etc. With luck you'll kill one or both of them - worst case should be that they're both heavily injured.

If you need to kill them off after the first phase of the attack, just run as fast as possible around the arena to spot them (recharge flasks on ads if you must). Take cover behind trees/stones when he does the wolf stream attacks and move out of the debuff that appear on the ground.

Mob density in Dark Forest is nice too, owing to the large, open, outdoor map layout. So well-rolled maps should yield very good map drops.

12 Vaal Temple Boss Kills:
No recommendation at this time.

Vaal Temple maps run about 50c each, and each map contains 3 bosses. Completion of this challenge will run about 200c.

However, Vaal Temple has been notoriously difficult in the past: this is a T16 map. If the Vaal Temple bosses are as difficult as other T16 bosses, completion of this map will be beyond the capability of most builds.

I haven't tried this map though so it could be much easier than I expect - hence no recommendation at this time.