There are a lot of good guides here, just search, but a few pointers:
In the first battles you can only do damage, somewhat useful because the damage you inflict increases (log2) with total anxiety. Focus on the chosen with least or close to a power of 2 in total anxiety. Sometimes it's good to extend the battle, but not always to maximize damage. Be conservative with spending evil energy. Try to spend it on developments that improve battle in general, extending it, giving damage multipliers. Wait with commanders a bit.
Once you have all Chosen, and have an extra capture and can extend battles once or twice then sometimes there's a way to really affect a chosen or two badly when invoking a commander. Commander upgrades are rarely useful in the beginning. Save before a battle, learn the keyboard shortcuts and write down what you do for the battle. I do it like below, the numbers are the keystrokes I press:
Code:
Day 7: Picked development: Weakness sense, battle with Commander, Target: Silence
1 4 3 1 23 32 3z 1z 13 12 14 2z 1z 14 13 11 12 31 33 33 14 14 14
=> I also write down the result of the battle; total anxiety per chosen, breaks and evil energy gained.
If it almost works, then there's usually a step that you can change that lets you overcome the hurdle. I load very often until I get the best out of the battle, but it may be more fun without.
Capture or surround chosen before the extermination is complete, inflict enough damage for them to gain another opening once they release themselves. The best schemes interleave one, two or twee chosen with surrounds and damage one in turn so the battle can continue. If you want to drive the chosen apart, make them less helpful to one another, try not to break their minor weaknesses