Template:Terestian Illhoof strategy
This encounter is fairly simple: Terestian is Immune, and can only be damaged through killing his Icky Imp (Heroic)s. Terestian tends to play pretty aggressively, and has some removal such as 
Soulfire, Bane of Doom and Corruption, as well as Doomguard to remove any persistent problems. Kil'rek (Heroic) makes an occasional appearance as a fairly substantial Taunt, making it harder to get at the Imps.
AoE damage is the order of the day for this boss. On Normal mode 1-damage effects like Whirlwind and Arcane Explosion will clear the Imps, while on Heroic you'll need something stronger like Blizzard, Consecration or 
Shadow Word: Horror, or repeated applications of Wild Pyromancer's effect. Aside from obvious AoE spells, minions like Abomination, 
Flamewaker and Tentacle of N'Zoth work well. Baron Geddon can serve very well as AoE damage, but he is likely to be removed, making him expensive.
Because the Icky Imps are "sticky" (functioning similarly to original 
Dreadsteed), over time the board will slowly fill up with them. This increases the pace and danger of the match, but also makes it easier to defeat Terestian, and makes AoE effects more rewarding.
Grim Patron works well for repeated trading, especially as most minions is this encounter have 2 or less Attack. Acolyte of Pain can be effective for the same reasons, especially on Normal. Minions like Frothing Berserker can be brought to huge levels during this encounter, but serve little purpose as the boss itself is immune. High-Health minions like Mogu'shan Warden can be effective for repeated trading, especially with healing available. For warriors, 
Protect the King! works very well, and combos well with Bolster. Fool's Bane can be effective, but can bring the warrior's Health dangerously low, especially on Heroic.
Mind Control Tech or other Mind control effect cards are excellent in this encounter, as the board quickly fills up and a stolen Icky Imp will endlessly respawn for you, while still dealing its damage to Illhoof. This allows the player to trade each stolen Imp into one of Terestian's Imps each turn, for an easy 4 damage per Imp per turn.
Knights of the Frozen Throne's cards 
Despicable Dreadlord and Defile make this fight particularly easy, because their effect deals exactly 1 damage without hurting the player's hero (unlike most other AoE board clears). Defile will straight up kill Illhoof, if there's more than one Icky Imp on the board (as it triggers 14 times, delivering 14*2*2 = 56 damage).
Heroic
Priest is an excellent choice for this encounter. As well as a range of AoEs, the class' healing abilities can allow you to repeatedly trade your minions into the Imps and then heal them back up; Holy Nova especially can be massive in this encounter. The priest's range of mind control effects also work extremely well in this encounter. Shadow Madness is the best spell for this - as long as the stolen Imp dies that turn, it will become permanently under the priest's control - while Cabal Shadow Priest also works well. Power Word: Shield works very well with Wild Pyromancer for even more AoE damage. 
Excavated Evil also works well in this encounter, and Shadow Word: Horror can cause tremendous damage in this fight and should be saved for near the end of the game when numerous Imps are present on the battlefield.
On Heroic mode, to set off the Defile chain reaction described above, you will need to do a single point of damage to an Imp (easily done with Mortal Coil) while a full-health Imp is around, such that there's always an Imp dying during each cast of Defile. 
Lord Godfrey, whose Battlecry is effectively a 2-damage Defile, will also easily win you the game.
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