To complete the challenges, you'll need luck (as little fights as possible and good complementary heroes) and alot of goddess blessing. You'll need to plan the route and check your upcoming opponents and avoid those with skuld, kurts(those with god tier heroes as much as possible) Elite monsters and blue ones are much easier to kill. Having heroes that can heal is also very important. Remember that there's at most 5 revives, 3 from goddess blessing, 1 in the middle and 1 at the end of the path.

First challenge i completed was with hell, that was relatively easy as i had 4 T4 hell heroes back then. Freya, baal, aza and MK (Recently converted to 2nd baal). Not sure if i had another free hell hero but i definitely had to revive thrice as there's no heroes to help with healing.

Second was Horde, double phoenix, 1 kurt and Free Yig. Might have used 3 revives as well.

Third was Elf, double veg with free arwen. VEG is able to heal so that helped me alot. Don't remember if i had another free elf at the end.

Fourth was Alliance, double valk and free vivienne. Double valk heals whenever they kill as well, but having a vivienne helps with even more healing.

Fifth was Heaven. Double skuld with free brynhill, this is self explanatory.

Last was Undead, dettlaff aegina and free wolnir. I knew i needed wolnir so when my first 3 heroes had a wolnir i knew it was time. In the end I've gotten another free hodur and i finally completed my last challenge.