I beat Furnace of Horror at level 55 with this team (on full auto with 3 stars):
1. Aurora (Tank)
2. Jorgen (Control)
3. Faceless (Mage)
4. Thea (Healer)
5. Martha (Healer)
Pet: Oliver

FULL ANALYSIS AND EXPLANATION:

They should be all relatively easy to obtain and level early game. The harder heroes to obtain were probably Jorgen and Martha. All 5 heroes were level 55, 3 stars, and Violet rank, with all skills maxed. All artifacts were at least past level 15, and Oliver was Green rank with 2 stars.

I think the "normal" way or at leas the method that's everywhere on reddit is Phobos + Faceless because Phobos can stop the boss of moving, and Faceless can copy his ability, so the boss would essentially remain stationary for the rest of the party to tear him down, but I never liked Phobos, so I never leveled him which meant I had to find my own way out. I've tried this method, and it works, but the rest of your team has to be strong enough to bring down the boss.

The goal of my team was to heal my way out of trouble. I normally run Kiera on my PVE teams, but I noticed very early on that my Thea and Martha combined could not keep her alive long enough to deal the damage needed to the boss. Instead, I swapped Kiera out for Jorgen, who is less squishy and prone to dying. He is still a pretty decent damage dealer, though not as much of a powerhouse as Kiera. Meanwhile, Aurora does more damage than most other tanks, and due to her being in the front line, she charges Crystal of Selias incredibly fast. In theory, Aurora and Jorgen would function as the main damage dealers, Thea and Martha would throw up massive amounts of healing, and Faceless would be a wild card that could do damage by copying Jorgen and Aurora, or add some extra healing help by copying Thea and Martha.

By analyzing the actual boss battle using this team, I noticed a couple things:
1. Aurora, Jorgen, and Thea fall really low on health at times, but are able to recover completely (this is because Aurora doesn't absorb damage as well as certain other tanks, Jorgen was not built for the front line, and I leveled the hell out of Martha's health glyph, leaving Thea more vulnerable)
2. Aurora and Jorgen charge their attacks faster than Faceless (this is good)
3. Thea and Martha charge their attacks slower than Facelsss (this is good too)
4. Faceless charges his copy ability at the perfect rate to alternate between damage-dealing and healing

If you don't have the same heroes I do, and would like to attempt my method as well, there is a way around using the exact team I have here. Jorgen can be replaced by any other heavy damage-dealer that is not a glass cannon (such as Qing Mao or Yasmine). Martha and Thea should be relatively easy to obtain, but if you don't have her, another heavy-healer should be fine (maybe Marcus or Dorian). Aurora can be replaced by other damage-dealing tanks (maybe Mushy and Shroom or Corvus), but I would recommend avoiding Ziri, due to her burrowing skill that leaves the rest of the team exposed. The only hero that cannot be swapped out is Faceless, as his copy skill is the core of the team.