How to beat Loatheb on heroic mode?
The gimmick behind beating Loatheb is quite simple and a really nice twist to the usual way Hearthstone is played. It is all focus on you taking advantage of the spore card
The popular YouTuber Trump accidentally stumbled into a great deck against Loatheb in the form a control Shaman deck (also using only F2P cards). The basic idea is to maintain a strong board while keeping his clear, this means when the removal cards come out and the spores are down, you can take full advantage of the massive damage boost. Shaman is paticularly strong because of the ability to play a totem almost every turn, as well as thing like Feral spirits to get 2 minions from 1 card; putting more presence on the board.
Another good deck would be a Paladin; the ability to 'get a guy' each turn will be very strong when combined with the spores. Maybe even a divine shield paladin that can use early creatures such as the Argent Squire and Scarlet Crusader to build board presence while possibly soaking the spore creating AOE with the divine shield, leading to a strong next turn.
Update:
I saw a great deck that can make outstanding use out of the spores against heroic Loatheb. First of all, a big problem with heroic Loatheb is the fact he starts with a 3/6 Fen Creeper on turn 1! That in itself can be too much for many decks to handle right from the beginning. It could take several turns to take that thing down and by then he has probably played a few more strong minions, the game is over before you can even kill a single spore! However, a great way to counter this is to roll a Rogue deck. Saps will sort that Fen Creeper right out.
Rogue is also very effective for the following reasons:
Sap is essential for either getting rid of the initial Fen Creeper and getting rid of bigger minions.
Fan of Knives is a great mass spore killer and the extra card draw in this deck will be very useful.
Your ability to create a dagger is also very useful for killing the odd spore which means you don't have to run a minion into it or spend mana on removal, meaning you can push more damage through to his face.
SI:7 Agent is a great way of getting a good minion out and popping a spore at the same time, ideally you kill spores when you have multiple minions out but using an SI7 Agent on a spore is like playing an 11/3 minions for 3 mana.
General removal like Backstab and Eviscerate is very useful for board clear or killing off more spores.
The Deck List
The Essentials:
2x Sap : Almost required to remove the Fen Creeper early, it is also possible with Backstab+Eviscerate but thats an extra card you need to draw so it's less likely (but arguably more effective). You will have the highest chance of winning with a Sap in your opening hand.
2x Backstab + 2x Eviscerate : Remove is vital for clearing minions that need to go or popping spores.
2x Fan of Knives : Fan of Knives is awesome for taking out a pack of spores, also drawing another card that is very helpful in a deck like this.
Every 1/2 1 mana card + cheap charge minions + (Stonetusk Boar) + Wisps!: This is the main theme behind this deck. Flooding the board is very important for knocking down his health, combined with the spores you can pump out a ton of damage using cheap, usually worthless minions.
A possible version of this deck : Anti-Loatheb Rogue Deck
The deck is quite simple and should be really cheap if not free. The basic idea behind is is to use as many 1/2 minions as possible. Why? Well, simply because your minions damage is practically irrelevant! The only major damage you will be doing is through using the spores so it is much more important to go for quantity of minions instead of quality.
Why 1/2, why not 2/1? Because he has removal in the form of a 1 damage to all enemy minions, so if you fill your board with 2/1's they all die. The great thing about using the 1/2's is that that 1 damage aoe removal also spawns a spore. This means that on your next turn, ideally all of the cheap minions you flooded the board with will be alive and there will be 1 or more spores just waiting to give you a board full of 9/1's +.
Another thing to note, while you have no control over it, Loatheb is awful at using his Faceless Manipulators! If there is a spore on the board and he decides to play a Faceless Manipulator, chances are he will use it on the spore. I don't think I need to tell you how that is not a good idea for Loatheb. I even saw him use a Faceless on a spore when a Sylvanas Windrunner was also on the board! (she is not part of this deck).
Your best bets are zoo decks with low cost creatures or decks like tempo warrior...You will have to take control of the board as soon as possible,so that when the spore comes in to play you have as many minions as possible to maximize the damage gain.
Read this for full detailed guide.
And here you can watch those wins with druid and warrior decks.