THE QUESTION:
This question is pretty vague, but I will give the best advice I can from personal experience.
MY CREDENTIALS:
In my 93 arenas, my average wins is 5.29 (I have done at least 6 runs with each class). I also have CCG experience from a long time playing Magic the Gathering.
ANSWER:
The best way to learn is to gain exerience. Learn the cards (all of them) and how they interact.
One of the fastest ways to do this is to watch videos of other people playing the game. If you go to twitch.tv, select "games" from the menu on the left and select Hearthstone, it will give you a list of people streaming their games.
I spent over 40 hours watching people play before I got my access. Kripp (kripperian) streams a lot and is a very good player. Sadly, he doesn't always speak through his thought process so it is harder to learn from him if you are not familiar with the game, so this is not the best place to start, in my opinion.
Another option is to go to Youtube and watch some of the arena videos posted by Husky (huskystarcraft) or TotalBiscuit (thecynicalbrit).
TotalBiscuit has ~20 videos of himself drafting and playing Arenas. I have found his commentary to be the most insightful as to card value when drafting. It is also valuable to watch them play with the decks they draft and to see how it all comes together.
These will help you to see what other people have done well and poorly and you will be able to better understand your own mistakes.
That advice given, here are some areas to be aware of tactically.
STRATEGY: Part 1 - Card Advantage
Card Advantage is king - One of the first concepts you need to understand is card advantage. Card advantage is gained or lost based on the efficiency of your cards as well as how you play them.
As an example, as a mage you could play Arcane Blast (deal 1 damage to all enemy minions) on turn 2 to kill his turn 1 creature with one health. But if you instead play out 2 minions with +1 Spell Damage and play the Arcane Blast on turn 4 do deal 3 damage to all those minions and get 3 kills, you are creating card advantage.
Card advantage is often referred to as going "2 for 1" or "1 for 1". If you used your Arcane Blast as in the 1st example in the last paragraph, you would have gone "1 for 1", that is, you used one card to kill 1 card. In the second example you would have gone "1 for 3" or "3 for 1" (it is often said either way, the point is 3 cards were negated by 1).
Knowing what cards can be used in what situation to generate card advantage is important to increasing your skill level as a player. Sometimes it is the best decision to do nothing (wait a turn).
Card advantage can also be thought of as "Number of card choices available", as in your handsize. In this sense, a card that draws cards can generate a powerful amount of card advantage sometimes by giving up Tempo. A great example of this is the Priest's Northshire cleric. A 1/3 creature played on turn 1 can draw a lot of cards if not answered quickly.
An example of a great play for card advantage here would be going 2nd with a Priest, you play a Northshire Cleric for 1, use your coin and follow it up with Power Word: Shield. Powerword shield makes your minion a 1/5 (hard for your opponent to deal with on turn 2), it also lets you draw a card.
Now if you opponent does have a spell to kill your creature on turn 2, they have gone "1 for 2" against you since you invested two cards already (plus the coin, which usually isn't counted). You did get a draw out of the Shield, but understanding how to make cards work together will help you become a better player.
STRATEGY: Part 2 - Board Presence
Strategically, you need to be aware of the concept of board presence. When you play an Arcane Blast that kills off 3 minions on turn 4 and you still have a 2/2 and a 1/4 each with +1 Spell Damage in play, you have captured board presence.
Certain cards help to generate board advantage more quickly, but trying to recapture board presence is like climbing uphill. That does not mean that the player with the most minions on the board is going to win, but you need to understand in what situations what cards are useful.
I would also like to tie the idea of Board Awareness to this. Generally it is considered sound strategy to control the board (prioritize killing your opponent's minions over dealing direct damage). However, there are a lot of situations where that may not be the correct decision. Even if you can not win on the current turn, if you can bring an opponent from 15 life to 5 and represent fatal damage for the following turn, that could be a better choice.
So when analyzing Board Pressence, be aware also of the players' current health.
STRATEGY: Part 3 - Health & Deck Types
In hearthstone you have to consider the value of life. Having more life than your opponent does not mean you are winning. Life is a resource, similar to cards (card advantage) and mana crystals.
A great illustration is the Warlock's ability lifetap: lose 2 life but generate a card (creating card advantage at the cost of life and tempo).
Some decks have a lot of cards that try to rush down an opponent quickly before they can start dropping big cards late game. Warriors have a lot of good synergy with this strategy from increased availability of Charge and also support from weapons.
Opposite of the aggressive style, there is also more of a control heavy style that uses a lot of spells to remove your opponents creatures. Prime examples of this would be a mage with +spell damage minions. These type of decks tend to need more support from Taunt.
Do not ignore the value of minions with Taunt. That does not mean you need 12 taunts in deck, but without any you may find you struggle whenever you lose board presence.
STRATEGY: Part 4 - Balance
With specific regard to Arena, your decks need to be drafted with balance. You want to have some minion kill spells and some taunt. It is harder to generate gimmicky effects, but synergy is still important to consider.
That said, there are some simple rules to follow.
- Picking a Murloc is a huge gamble b/c of how that race works. You need to get a lot of them to synergize well together. That does not mean that it is the wrong choice to include 1, but be careful not to commit yourself too much to one idea.
- Picking a Pirate with a weaponless class is almsot always a bad idea, but not necessarily the worst (for example, given the choice between Dread Corsair, Wisp, and Argent Squire, sadly the best pick may be the pirate (this happened to me recently).
- Learn the class specific cards (Which classes have weapons, what are they? Which classes have secrets, what are they? What is each class' "kill spell")
- Make sure to have some taunt and a way to kill minions.
- Silence can be very valuable! Ironbeak Owl is not that great of a card, whereas Spellbreaker is one of the best.
- Spells / Abilities that heal are easy to over value, especially if they can only heal your character and not a minion. That same can be said of spells that can only deal damage to an oppoent but not to minions. Learn how to value cards well and you will become a better drafter and deck builder.
Good luck! I hope this helped.
Best Answer
I haven't been able to find anything recording the number of wins but Gamepedia does have some good info here: http://hearthstone.gamepedia.com/Arena#Statistics
Out of 8192 players 53 will attain 12 wins. Making 12 wins a 1 in 155 chance or about 0.65% chance.
This is compared to the Ladder in which [0.5% of players reach legendary each month].(http://hearthstone.gamepedia.com/Play_mode#Ranking_percentages)
These statistics are still not equivalent however, because the number of players entering the arena each month is unknown. Even if it were known it could be that one player reaches 12 more than once per month, skewing the number of 12-win players.
As far as "arena is getting more difficult", this is a judgement of player quality in the arena which would be difficult to measure. However the skill of individual players will not impact the number of players reaching 12-wins. For every loss someone else won. When two 10-win players face eachother one must make it to 11 wins. When two 11-win players face eachother one must make it to 12 wins.
It is possible that the wins have become more concentrated? For example if a single player reaches 12-wins several times in a month that lowers the number of 12-win runs available to the general populace.