Combat is relatively simple.
On a draw i reroll, original rules state that the defender wins but that would mean players losing vast amounts of soldiers to mutants and bandits (there were a ton of draws in the background).
Basically you have 1v1 dice comparisons. More units only mean that you get to roll more often.
As for different unit types:
Against the "AI" i look for unit comparisons that favour you: so you will always attack witht he armoured car, then the soldiers, then the rangers, to preserve as many of your men as possible.
PvP battles work differently: as mentioned in the beginning, there will be an order of precedent to orders once it becomes relevant. That means that one of you gets to move first before the others. That means that player if he attacks also decides in which order to attack.
Example: Neomakhnovia with 3 Soldiers fights the FOurth Reich with 1 AC, 2 Soldiers and a Ranger.
On your orders you both specify that you want to storm the other players station. I roll and it turns out its Neomakhnovias turn first.
In your orders you already specified to target the ranger and soldiers first, then deal with the armored car...and so i roll.
If it the Fourth Reichs turn first, i will do his orders and specifications first, so for example he decides to exclusively use his AC until it loses.
All in all an ideal order for combat against another player should contain:
who goes where,
who attacks whom with which preference
and retreat orders (if possible or wanted)