If you ask any professional player what the single most important skill in an arena battler is, they will all give you the exact same answer.

Playing the game without knowing how much elixir your opponent has is like playing poker without looking at the cards on the table.
The Economy of Aggression
Every single aggressive action you take in the arena should be dictated primarily by the opponent's current bank of elixir.

You create an advantage by making positive defensive trades (e.g., using a 3-elixir Arrows to kill a 5-elixir Minion Horde).
Play safe until you know the math.If you are up by 4, launch your win condition immediately.If you are DOWN in elixir, play hyper-defensively. Practicing the Count
You must train your brain to do it subconsciously, starting with simple approximations before moving to exact numbers.

When they drop a heavy card, like a P.E.K. If you have any sort of questions relating to where and ways to make use of tower rush, you can contact us at our own web site. K.A (7 elixir), tell yourself "They are now Low Elixir."
The Match StageThe MathHow Hard it is to CountSingle Elixir (First 2 Minutes)1 Elixir every 2.8 secondsEasy to Moderate; plenty of time to calculate trades and track generation in your headDouble Elixir (Final Minute)1 Elixir every 1.4 secondsExtremely Difficult; the math changes so fast you must rely on instinct and 'feel' rather than raw numbers Mastering the Economy
Tower health is a deceptive metric; you can have full tower health and still be losing the game if you are down 5 elixir.

Knowledge is power.