Well, even though i found the chicken launching hand mortar and other nasty legendary toys (purple shotgun, scythe, hammers...), i find it very hard, if not downright impossible, to beat map challenges that involve bosses. Rachnoss and Vail the Immortal, specifically.

Does anyone have some advice, or even better, link to some guide for those two?

Challenges are, well, challenges and need some preparations.
Done with transmutation
-can add third property to weapon if used with legendary demonic power
-can upgrade demonic powers to legendary when you upgrade main property
-can upgrade raw weapon damage
-can upgrade suffixes
(you need a lot of different demonic powers, demonic boss drops them, or you can farm collector's house - it is small and contains 6 champions with random powers)

First thing to create
1. Farming weapon http://i.imgur.com/cE6kJci.jpg Elder Weapon of extraordinary luck, with added "of luck"
Weapon type doesn't matter (this is rare combination), but preferable mortar (rocket jump for mobility and vulnerability)
Elder -> more renown levels-> more chests -> more purple chests -> more items.
Extraordinary luck - better items, legendaries instead ofyellows, greens instead of whites.
Luck - selfexplanatory.
2. Killer weapon. Because there are weapon-related challenges, you need every item type.
Example, sword http://i.imgur.com/RST5m1F.jpg
(Personally, I would recommend to start with swords - card "Blademaster" gives 4% hp regen for 20 sec after crit with sword ability, third sword hit adds vulnerability - guaranteed crit, makes all "stay above XX%" challenges a joke.

What do you need to get?
1. Rabbit's foot http://i.imgur.com/QIfcyqP.jpg
100% crit for 5 sec,
sword's Q refreshes on crit. My sword's Q crits for 2700 and can dish out 7-8 Q in these 5 secs.
2. Hexes add armor. You need to get armor penetration.
two variants - Card "The Devil" (20 arpen per Wicked card) or "The beast" (90 arpen, divine propert can add more, I have one with 22). In the long run Devil would be better - there is outfit which lets you use 6 cards - 120 arpen. Beast - 110.
Same Destiny points, plus additional Wicked property on the devil.
3. Fill the rest of destiny cards. Each weapon type has 3-point wonder card, only Hammer's one is situational.
So, Devil 4, "Weapon card" 3, Vampire/Bishop 4 for healing (depends on weapon/battle/damage output, can you facetank with lifesteal or should kite and critheal on someone like Gabriel while having regeneration form sword); wildcard 8,(19/20 points by now).
Last destiny slot(s):
With Adventurer's outfit (+1 card), The seer (+4 points) and pick something. Depending on challenge types, Tower, Alchemist, Runner... Many would make sense..
Without... There are variants. Probably the seer and upgrading Bishop to 8 would make sense. Maybe alchemist 1. Maybe downgrading healing and getting runner...

The easiest way to get cards - Purple chests (you get them by transmuting from yellows (them from greens and them from whites, which you get per level), usually 3-4 divine/wicked cards per chest. Cards:weapon ratios in chests is like 6:1

Result::killed Rachnoss with 5 hexes in 120 secs with this sword.http://i.imgur.com/aLAFRLn.jpg
For Vail I would recommend to rush right to phylactery, destroy it from the broken bridge with Mortar's fire lake and Boomerang (safe from gargoyles) then kill Vail.

Thank you, what a nice compilation of a guide! Truth be told, i already had the farming weapon and the rabbit's foot on me, farming weapon from way back (found it in a mine from a spirit champion). For main i use either chicken cannon, or Vera the purple shotgun, now sometimes a purple sword called storm or something (for crit and weapon skill, feels like cheating).

I recently found a Yellow Lightning gun, which is cursed (many more damage) of the assassin (more chance for crit) and i already maxed its damage by feeding it yellow weapons and its crit chance, by feeding it destiny cards. I also added to it the Efficient, for shorter weapon cooldowns and i am working on maxing its magic properties. That thing is a killer; with "Lightning" purple destiny card, it is a force to be reckoned with. Rachnoss is done, but Vail has those two gargoyles guarding his phylactery and they are of the extremely high damage with beam, types. I still struggle on this one.

For destiny cards, i have: Legendary Wicked Seer, Legendary Strength (crit chance), Legendary Lightning, Legendary Wicked Rogue (crit damage) and Legendary Divine Bishop (health on crit). Works like a charm and the Lightning Gun deals heavy damage...

http://i.imgur.com/KIEcUN7.jpg
This is place where you can safely skillshoot phylactery, skellies is the source of overdrive.
You can get there by Rocket jump or by falling into lower floor and walljumping to the other.
Mortar and Boomerang would be your friends.
Personally, I killed these gargoyles within one use of rabbit paw (3q on each)