Template:Giantfin strategy
This is a fairly straightforward fight. Giantfin will summon a large number of Murlocs, which are mostly weak but with the help of the usual synergistic Murlocs can grow in size. However, the main threat is Mrgl Mrgl Nyah Nyah (Heroic) and 
Anyfin Can Happen, each of which can suddenly threaten to overwhelm the player. Note that several of Giantfin's Murlocs have Charge, making them able to attack immediately after being resurrected by one of these effects.
At the start of each turn, the boss' auto-cast Hero Power Mrglmrgl MRGL! (Heroic) will try to draw additional cards; in Normal mode the number is dependent on the size of both heroes' hand, making it a good idea to keep yours fairly empty.
Normal
Since the boss draws every turn until he has the same amount of cards as the player, it's useful to keep your hand as empty as possible. Therefore it's recommended to play an Aggro deck with lots of cheap minions that can fill the board and keep the hand empty early. Examples include Zoo Warlock, Aggro Paladin and Face Hunter.
Heroic
An easy way to beat Heroic Giantfin is to use a mage deck full of AoE cards: Neutral minions that play into this strategy include 
Explosive Sheep, 
Unstable Ghoul, 
Abomination, 
Tunnel Blaster and 
Primordial Drake.
Spells include 
Shooting Star, Arcane Explosion, 
Flame Ward, Flamestrike and Blizzard. They can be improved by minions with Spell Damage as well as combined with 
Flamewaker to remove minions with bigger health values.
Since the boss possesses many Murlocs in his deck Hungry Crab will fit perfectly in the strategy.
The main goal of this is to hold the boss minions as long as possible until he has reached fatigue, since Giantfin obtains 3 cards per turn from Mrglmrgl MRGL!, his deck will be out of fuel pretty fast even possessing 51 cards in Heroic mode.
Coldlight Oracle and Research Project can speed this up. At the same time this card can also burn the enemy cards by making the enemy hand full if played early; this tactic becomes even more effective with Frost Nova since the boss can fill his side of the board pretty quickly.
Bringing Counterspell can block the enemy revival spells, and, if it counters Anyfin Can Happen, Giantfin will have lost almost his entire turn. Ice Block and Ice Barrier are other useful secrets that can stall the game for the player. To bring out these secrets with tempo it's wise to bring Mad Scientist and Arcane Flakmage.
Since you want to stall the game and most enemy minions are low HP it's useful to bring Taunt minions with low attack but high health values. Examples are Deathlord, 
Ancient of Blossoms and Mosh'ogg Enforcer. The boss will have a hard time getting through these minions.
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