ALRIGHT
OPENING OF THE GAME AND Y OU!!
So when you start a match, the most important thing is to look at your options, and wait. There are various openings depending on what type of deck you're running, but we'll get into that later. The first thing you want to do is wait. Save up your Elixir. Only react to an opponents move. If you're getting full to Elixir, try dropping a slow unit behind your main castle to buffer the time of your opponent dropping a card. This is also advantageous to you, I'll go into it. Now, your opponent my try to all in a lane or wait to drop units like you are. That's no problem, because if you drop a slow unit, you're maximizing on your elixir and your opponent isn't. If you react, you're maximizing on your elixir and your opponent is it. The only way to put yourself in a disadvantage is if you drop a unit to try and attack one of their towers. This tips the scale into their favor.
THE SCALE, TOWERS, AND YO U!
Think of the map of the game as a scale. When you put a unit on your side, the scale tips into your favor. But the second that unit crosses into your opponents side, then they have the advantage if they can react to this. This is the Tower buffer.
THOSE NEFARIOUS TOWERS!
There are two ways to approach the tower. One, they are something you have to defend. Or two, they are there to defend you. I prefer the latter, since offense is greatly rewarded in this game. So what is the tower buffer? It's simple, when an opposing unit crosses to your side, they start getting pelted by arrows. They may even attack your tower! But this is ok. You want this, because it gives you unit advantage. If you drop a ranged unit behind your tower, then you're able to cost efficiently take out their unit and you end up having the elixir advantage on your side of things. Not only that, but because units tend to stop to attack, that means you can create a huge wave of units on your side, while their units died by crossing to your side. So use your tower offensively, that is, if it's full health don't be afraid to let your enemy attack it, even if it's with a prince or pekka. As long as you gain the advantage on units, then it's worth it.
SCALE BACK TO THAT SCALE.
This is all represented in what I like to call the 'Scale'. Think of a physical scale, evenly balanced, only moving when a unit is dropped on a side. If you have a unit on your side, your scale is heavier and thus stronger than the opposing side. If your opponent has a unit cross over to your scale, then you still have the advantage because of your tower. The trick to being able to overpower their side is to have them throw units at you, and for your units to overpower them and them spill over to their side.
For example, let's say they throw a giant. The second it hits your side, drop your units next to your tower so they have to walk up to the Giant, and don't put them close to the giant because they can counter with their own units behind the giant. The Giant will probably take a bit to take out, so you're growing elixir while your own units are attacking the giant, meaning when he goes down you can start an offense with your stack of units. Because a giant takes a while to take down, that means that you have time to still have elixir reserves to enforce your offense and overpower their tower and their reactions.
KNOWING YOUR OPPONENTS DECK
It's also important to try and 'count' your opponents deck. Each deck has 8 cards, and in a match you tend to use each card once. Low costs let you cycle faster, high costs can be a bit harder to predict but can prove advantageous. Either way, knowing what your opponent can drop can help you tip the scale of power. If they just used a fireball, chances are they can't react with another fireball immediately unless they have a mirror. Dropping your Barbarians right after a fireball can be better than blindly dropping them even in a defensive position. Sometimes you can try baiting card drops, IE you have a Knight+Musketeer combo you want to use, but dropping your goblins and skeletons might scare him into using a fireball.
Knowing the costs of cards is important to in reacting. If they drop a PEKKA behind their castle, then you know they just used 7 elixir and only have 3-4 elixir to react, or if they dropped a Golem then they just used 9 elixir and are open to attack. Keeping mental note of values is helpful in knowing when to counter attack and where. Dragon And Prince is 9 elixir, Skele Giant and Musketeer is 10, etc. If you know you have a response ready for when those units come to your side, then dropping a Prince on the opposing side can effectively win you the game.
OPENING MOVES AND Y O U!
This is the deck part of the strategy. There are various decks. Low, high, mid cost, hut decks, giant decks, zerg decks, either way what kind of deck you have will determined your opening move, and to an essence what cards you have in your starting hand. If you have the Elixir card, usually you want to drop that immediately and drop it in front of your castle. The first reason is because you want it to start producing elixir immediately. The 2nd, if they make a mistake in a fireball or Rocket, they might end up hitting your castle, thus activating its defenses and passively making your towers stronger because it aims that far.
For hut decks, opening with a barbarian Hut is common, but you might want to wait until you have a goblin hut out to make the most of it. Hut decks run counter to a reactionary deck, in that it's always attacking one side, and it can build up if your opponent is playing reactionary. In essence-Hut decks counter reactionary players, and reactionary players have to play offensively to counter a hut deck even if it seems disadvantageous.
As for other Card openings, dropping a fat slow unit behind your castle when you get close to 10 elixir lets you remain defensive while telling an opponent "I have this here, are you going to react to it?". So let's say we drop a PEKKA that will go down the right side. Your opponent will either save cards to stop him, or immediately attack your other side, in which case you can still reaction and put him at a disadvantage. But it also gives you time to build elixir as the PEKKA travels down, and once it reaches the river crossing you can drop heavy support on it OR pull a fast one and attack the opposite lane at the same time, and this can cause your opponent to make mistakes trying to stop both or try and all in that PEKKA, giving a free opportunity to take a tower. This type of strat can be used mid game also, dropping a slow unit behind your castle 1 minute in can be advantageous because by then you know what your opponent has and can counter it, and it lets you create a ball of units.
CARDS AND DECK BUILDS
So let's look at deck builds. A deck with Golem or PEKKA needs to be supported by either The Elixir Generator, or low level cards like Goblins, Archers, Knight because you have to be able to react and support your PEKKA. Bomb Skeletons are powerful for their cost, able to wreck groups of barbarians if placed behind your pekka. Goblins can do work either at range or melee. Usually a fireball or arrows is necessary to be able to counter minions, since your anti air will be your weakest part.
Giants are good as meatshields, and common in mid-cost decks, but their effectiveness varies. Skeleton giants are good for having a big explosion at the end, and good at countering Pekkas or Barbarians because of that. Regular Giants are great if you can build a ball of units behind them, but because they're single minded they'll die fast if you can't defend it. Royal Giants..................we don't talk about them. Other mid-cost supporting cards are hog rider, Prince, Baby Dragon, Witch. All good for specific things, either gunning down a turret, creating a threat that can deal major damage, countering mass units, or creating units whilst having decent aoe spam. Witches can be treated like heavy units, in that you spawn them behind your castle so they can build up skeletons. Princes you want to use to counter assault a lane, baby dragons in conjunction with an air balloon can devastate a lane if your opponent isn't careful, and hog riders are good at diverting an opponents attention from another lane or doubling down on taking down a tower.
Low cost decks tend to be about the unit spam, capping out at barbarians or Giant, and having cards like Knight, bomb marauder, skeleton army, goblins and archers as well as minions. In conjunction with freeze or rage you can deal high damage extremely fast and unexpectedly.
Some staple cards are arrows and fireball. Able to counter low cost units immediately, at an area is great. Fireballs are good for the slightly heavier units, like Barbarians, huts, or towers. The lightning spell is also good to have for taking down towers or huts, or mid-cost units like Valkyrie/minipekka. The Zap spell is decent for stunning and at a low cost, it's also good at taking out skeletons or lower health units. It can also stun magma towers, resetting their damage gains.
Spell cards are useful in any situation. Freeze, Rage, and Poison are all great. Personally I'm favoring Poison right now, because it's a huge area of effect, it slows, and does damage while units are in it. Freeze is good in rush decks, or in combination with hog rider. Rage is good in decks where you can get a little mass of units going, but I don't see it as much. Lightning and Zap I mentioned above.
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So that's it. Go out there and get trophies n' stuff!
For specific deck strats, post your deck and I'll give you a rough idea on what to open with/how to react/ and any suggestions.