Combat Rules:
Your goal is to find a battle order that uses the groups' strengths and weaknesses to your advantages.
Swords counters Bows, Bows counters Axes, Axes counters Swords.

Combining:
You can combine two groups of the same team to change their type.
The new group keeps the type of the biggest group (or the destination's type if they're equal).

Attacking:
When two opposite groups meet, they subtract their numbers.
Each type has the advantage against a certain other type(Swords counters Bows, Bows counters Axes, Axes counters Swords), giving them +50% strength (meaning that 2 soldiers can eliminate 3 opponents).

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The idea of Combat Phase seems simple, however, it's much deeper than you think, actually. In fact, most players do not really play the Combat Phase to the best, or almost best.
Let me add more tips here, and I'll also put videos to show some situations/answers so you can know what's going on.

The basic is to take the highest advantage to all (or most) enemies while doing this, however, even if you think you've already achieved this, you may still haven't done it perfectly, let me explain them now.

Alright, let's get started.

Assume that we have the advantage on soldier type, if we eliminate an enemy squad(and the squad we use still haven't reach zero), but the number of the enemy squad we eliminate cannot be divisible by 3, we'll suffer from a tiny exchange penalty.

For example:
1. We use 60 swords to eliminate 31 enemy bows, we'll spend 20(2x) to exchange 30(3x) first; however, the enemy still has 1 bow left, you'll be forced to make a 1:1 exchange for it(instead of 1:1.5), that's the tiny penalty.
2. Now assume we use 60 swords to eliminate 44 enemy bows, we'll spend 28(2x) to exchange 42(3x) first; however, the enemy still has 2 bows left, you'll be forced to make a 2:2(because 1 can only exchange 1.5 most of times) exchange for them(instead of 2:3), that's the tiny penalty, too.

In conclusion, one of the tip is try to make an enemy squad can be divisible by 3, when we eliminate it on type advantage.
There's an important note here: (Assume that we have the advantage on soldier type.) If your squad is eliminated instead, and the enemy squad you attack still has units left, you won't get the penalty from this scenario.

There's a situation that can make you get tiny 1:2 exchange bonus.
The conditions are:
1. Your army type is in advantage against the enemy's.
2. The number of the ally squad must be odd(not even), assume it's 2x+1.
3. The number of the enemy squad must be at least 3x+2, or more.

At this situation, after you exchange 2x allies for 3x enemies, you'll have ONLY one troop left; however, since the enemy's type is being countered, and the enemy squad still has at least 2 or more troops left, it's impossible for the enemy to defeat the last ally unit with only 1, which will force enemy spend 2 for the last 1 ally instead.
Therefore, you can get 1:2 exchange ratio instead of 1:1.5, that's the tiny advantage you can try to get on Combat Phase.

In Conclusion, there are two tips for us to follow to make the best result on Combat Phase:
1. (On unit type advantage)When we want to fully eliminate an enemy squad, always try to make the number of that enemy squad can be divisible by 3.
2. (On unit type advantage)When the number of our squad is 2x+1(odd), and the number of the enemy's squad is at least 3x+2 or more, we can get tiny 1-exchange-2 advantage.

Now let me officially show some videos and more information below.