Hell's Realm

Tips

The biggest problem in this battle is the boss's plague spell, which will deal 20 shadow damage to all player's creatures after your next turn ends and it will stay active for the duration of 10 turns (i.e. it will deal 200 damage in total). Therefore a priest's Dispel is the single most important spell for this level. Be sure to have at least two priests with three Dispels each in their decks and make sure that each priest uses a deck with less than 30 cards. Around 22-25 should be fine. How to deal with the plague:
-The boss has exactly five plague spells. Note that he often casts two instances at once, sometimes even three. Since Dispel can dispel only one instance, don't waste it if you don't have enough of them ready.
-Be sure to tell your comrades early on when you have drawn a Dispel, since you might want to discuss on where to place that 3x3 spell before casting it and 30 seconds are short. By Shift-clicking on a card it will appear in your chat message text field.
-Desperate Soul is immune to plague, so be sure to use this creature. Note that there can be only one per team on the battlefield, so don't add too many to your decks.
-Creatures with the ability Soulbound have a 50% chance to return to your hand after dying, so about half of them will be able to return to the battlefield after dying from a plague.
-Nimble creatures are not safe from this spell, because it's not a single-target spell.

At first, the boss will summon a whole lot of Imps, namely Filthy Imp and Hellish Imp, both of which have Infiltrate and Panic Strike, with the occasional Jaraax the Tormentor thrown in. Therefore try to heed the following pieces of advice:
-Always place two creatures in front of them to make sure they stop at your frontline and eat the damage you've prepared for them.
-Use a heavy tank with Armor 2+ on the front line. Those imps have an attack of 2 (normal) or 3 (hard). You can use the various lizards that are available in the Beast reputation shop for cheap and/or some armored humans. A lizard's Lightning Breath is a very handy ability as well.
-Since some imps will always get past the front lines, be sure add creatures with Vigilance to your deck. For example Stormy Disciple is a great choice, because every single one of her three abilities are very useful. She's available from the Dragon reputation shop.
-Shadow Dragonkin works really well against creatures with a low attack stat and a high life stat, too. Make him BIG!
-A Werewolf Flayer behind a tank can become really strong, especially on Epic. If you feel like trying him out, put him behind a tank with a low attack stat so he gets the kills. Mind his low initial life though. He must be able to kill a creature the turn he's being summoned or else he will likely die in vain.
-Creatures with the ability Skeletal won't die from the plague. They will merely fall into a bone heap. As long as they don't get attacked the following turn, they will recover just fine. They also make for nice tanks.

In the second phase, the boss will attack you with creatures it killed, i.e. your own creatures. This is another reason why lizards are a good choice, because they are lumbering so slowly and they don't deal much damage. Be sure to use some creatures that can deal with them (i.e. creatures that deal magical damage or have Sunder Armor).

During this second phase the boss will also summon a lot of Succubus creatures with the Battle Fatigue ability. Deal with them this way:
-Focus some nuke spells like Smite or Slash or some disabling spells like Petrify on those with Battle Fatigue 2 and take care of the rest of them the hard way.
-Put some creatures with Savior in your deck, preferrably Angelic Aegis, because she has Vigilance as well. One might consider putting Angelic Savior in there, too, but beware that creatures with a high countdown are difficult to handle when facing creatures with Panic Strike.
If you have some Dispels left, feel free to un-charm some of your creatures when necessary.

Like always, Blessing: Sanctuary is an extremely powerful tool to protect units in whatever strategy you use to defeat this boss.

Because of the plague, you might want to refrain from using too many creatures that power up slowly, like Battle Priestess or Repeating Crossbow Captain, but once the plagues are dealt with these creatures can shine, too, so adding a few of them might work well for you in the end.

Correspondingly, creatures which are strong in the beginning and weaker later on, like those various Feles creatures (Feles Swordsman, Feles Assassin, Feles Assassin Master), can be used to quickly dispose of some imps and they aren't much of a loss when the plague happens later.

Meaty tanks like Werewolf Flayer or Fenris the Butcher are a viable choice as well, just so long as you feel that you can handle the Blood Price.

Note that when you try to follow all of those points of advice, you will probably have too many creatures for your deck. This is just a guide, not a walkthrough, but if you follow it roughly you should be able to beat that boss.

Source:http://rise-of-mythos.wikia.com/wiki/Hell%27s_Realm