How does combat work?

[Novaliz]

when i have a stack of units, i noticed that only one unit fights and then the next etc... Is it alway better to pick the unit who starts the fight by my own because i also noticed that when i attack with the whole stack the weaker unit does the first battle?

[Xilmi]

I would generally try to avoid stacking as this makes your stack an easy target for Nuclear-Missiles and the likes.
Also before attacking you will usually want to get a flanking-bonus. You get it when your units stand at least 120° on the other side of the unit you attack.
You can get 2 flanking-boni at once by surrounding the enemies first.

The order of attack also is quite important. If none of your units will win with 1 attack, you shall first soften the enemy up with the most expandable unit and then finish it off with a stronger one, so it gets the Experience for the kill.

If you have so many units that you still have stacks even after surrounding, you should think about getting better military-techs and build more expensive units. While it's harder to flank with them, simply because they are lower in number, their big advantage is, that they survive fights other units wouldn't, can retreat and come back after healing up.

Extremely high-end-units can get extremely powerfull and extremely expensive. You won't have big Stacks of 400+ -Strength Engame-Mechs.