Here’s a video of how I completed Legendary without using 5* heroes, and with only a few minor mana potions just to be safe. I’m very happy with how this went - I won on my first try and with a pretty mediocre board, meaning anyone could win this way with the right heroes. I plan on recording an explaination of my strategy once I have some free time, but thought people would want to see the footage during the event if they need help completing it.
I ran 2 blue and 3 green heroes since I wanted to kill Rumpelstiltskin first, and since the minions are blue and red I used 2 healers - Kiril and Melendor - to outlast the the damage dealt by enemies, plus Grimm, Little John, and Halsel to control the bosses’ mana while dealing heavy damage. Before the fight I focus on how I need to use each hero.
Kiril: Heal and protect the other 4 heroes who are all glass cannons, while also increasing their attack.
Grimm: Amplify the damage dealt to all 3 enemies per wave while hitting like a truck.
Melendor: Remove Boss Wolf’s riposte and provide a large heal. (Caedmon or Sonya would work as well)
Hansel: Cancel the special of whichever boss I am focusing down at the moment (Rumpel -> Wolf -> Red)
Little John: Slow the mana gain of all bosses while hitting like a truck.
At the end of the 2nd minion wave I charge all my specials and spend a lot of time removing the 3 colors I’m void in (purple, yellow, red) so that my board is as potent as possible for the boss wave. I then kill Rumpel first since he is the only real damage threat, letting me grind out a long fight vs Wolf and Red Hood without fear of losing any heroes.
The fight actually begins at 1:38. Sorry that the sound cut out early in the fight.
Hope this helps everyone trying to complete Legendary!