This is an event-challenge avaiable for everyone, BUT the suggested minimum level is 25+. The more the better.
The battle contains a lot of elemental damage, flying creatures, and a Crowned Nid (more on Hard mode).
The special tricks appearing here are: portals which spawn Lions for 2 turns, Firewall what appears close to the boss dealing massive damage anything there for its duration in every turn, and magma fields dealing 5 damage whatever stands on them in each turn until it lasts.
Magma Fields cause damage at the end of the player's turn, so if your creature can walk off it, you can safely put it down on it. Magma Fields are also welcomed during the first part of the battle burning away the first row of the Lions. Too few Magma Fields, or they not appearing right before the Heroes might cause the loss of the battle for the players, especially for lowlevel tryers.
Spawn Portals last for 2 turns, what means if the spot they appear is unoccupied, and the Lion spawning can walk off it, 2 Lion will come out of each. These portals appear 4 times during the battle, the last wave usually spawning right before the players' Heroes. It important to be able to block the portals, at least for their second turn - this can be achieved either by sending a creature there, by blocking a creature standing on it, or placing a Wall above it. This latest scenario is especially important to note when the portals appear on the 4-5th hex from the Heroes (not creature spwaning spots), and your creatures are prevented to reach them. therefor at least 2 Wall cards (any, but big HP walls are preferd, just like Human Walls - which can block flying).
As the portals will appear right front of the heroes, whose units will likely burned away by Magma, it is important to bring Backstab and Vigilance creatures to this fight (4+ in the deck, prefered with high HP for vigilance units to be able to stay alive longer).
Resistance units and/or big damage units are also important, as you have to kill fast, and be able to stay alive against elemental damage.
The lategame problem is Crowned Nid, what is likely to happen with stats like 70/50. This is impossible to defeat. When it appears, instead kill any other units, as the criter gains its power from them. If you kill the other beasts, you'll have no mre problem dealing with the sole Nid.