Oftentimes, the best all-around formation is simply putting all of your troops as close together as possible. Do this and it will take the enemy far longer to start shooting you compared to if you draw them all out in a line, and when they are within firing range, you will be too, and all of your firepower will unload on them at once, making for a much quicker kill.
When you attack someone else, picking the lowest-ranked player to attack is generally the best way to get an easier player. Higher-ranked players are almost always tougher to beat, but they will give you slightly better rewards when you beat them. They will often have more obstacles that you have to watch out for.
The toughest formations to fight against are the ones that are shaped like a giant arrow, with the leader in the back. Staying in the middle is absolute suicide, so when you suspect that the big arrow is coming (I.E. when you see incoming troops straight to the top and straight to the bottom), dart to either the top or the bottom so you can ambush one of the arrows. Then make as quick a work of them as possible, dart back up and take out the leader of the other arrow formation.
Scout the base that you are about to attack to see what kind of obstacles they have waiting for you. If you see a geyser, for example, then watch the timing of it and don’t be anywhere near it when it goes off. If there’s a cannon combined with some mines, then intentionally set off the mines by running over them fast and in a straight line (but don’t get blown up) so that you have more room to go to avoid the cannon fire.
Want to add more than just the beginning 100 to your defense spaces? Keep gaining levels and look for new skills to unlock. Some of the unlocks will allow you to put more defense into your base, while others will open up brand new spaces for you to fill.
Scroll sideways when you go to the skills menu, because there are far more skills than appear on the initial screen that you open. Many of the skills will be categorized under different colors. You can still unlock them no matter what color you are, though. The categories just mean that they have a different set of effects.