While the Frostwolf Warlord may not be all that great in constructed decks, I highly consider it a bomb card in arena play.
If I see this card, I will almost always pick it. Your opponent needs to have something that can remove it or stall it long enough to take it out of play and it'll trade favorably for you in most conditions. It costs 5 mana. If you manage to get it out during the mid to late game stages, you're bound to have a few cheap minions up. All it takes is a few minions for it to turn into a hulking monstrosity that will clobber anything in its path. The fact that Frostwolf Warlord is a part of the basic set means that it becomes immediately accessible for just about any deck. Great card to include if you're just starting out any need to add some much needed muscle and firepower to your decks. I've encountered the Warlord mostly in paladin decks because it synergizes extremely well with their hero power (Generate a 1/1) but it also works well with shaman decks.
Battlecry: Gain +1/+1 for each other friendly minion on the battlefield.
Playing with Frostwolf Warlord
The Warlord is especially effective in any heavy minion decks like beasts or murlocs. Basically, if you have a ton of minions, the Warlord becomes that much better. Hold some minions in your hand though in the event of any board clears like a Blizzard or an Explosive Shot. You can drop your cheap minions first then hit the board with the Warlord after the fact. Some of the other decks I've seen run it in tandem with cards like Dragonling Mechanic or Murloc Tidehunter (essentially, two minions for the price of one card). The battlecry effect is just a bonus. If you have no other minions in play, you can dump it on the battlefield as a simple 4/4 and it'll be able to hold off anything coming toward it. Given the choice (and if the situation warrants), you're almost always better off waiting until you can get some minions to help buff it up.
Playing against Frostwolf Warlord
Blow it up! Hit it with a Fireball! Maybe a Kill Command! A Lightning Bolt! Whittle it down to size with your spells so that you don't have to trade it with your minions! No removal? No problem! Say hello to my friends Spellbreaker and Ironbeak Owl. Using any silence effects immediately removes the Warlord's Battlecry effect and turns it into a mortal-sized 4/4 (which is still a threat but becomes much more manageable). No removal or silence effects? At that point, it turns into a race. You need to beat your opponent before you get beaten down by the Warlord. Lastly, keep an eye on your opponent's mana. If you're anticipating a Warlord coming out, consider dropping an AoE spell and clearing the board. You might not be able to stop it from coming into play but you can make sure it doesn't come onto the battlefield buffed!
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