I feel for mobile-only players trying to learn combat. Here's a link:
Mykan's Fighting Guide
Though written when there was no mobile and before Spire and the current tourney structure, the information regarding troops, their placement, and which units they work best against and are vulnerable to is still valid. Scroll down in the first post to just below the Battle Pentagon to start reading about some of that.
The map province fights still work the same way, so the info on the mix of enemy troops and recommended units to use will still work for those fights. The breakdown by province type starts at this post:
Ex: if you're fighting a marble province on the world map, the info under Marble provinces will be helpful.
My brain didn't like the 'pentagon' way of explaining things, and I was only able to really grasp the concept by looking at the 'Army Camp' tab of a military production bldg. Mobile has that window so maybe this will help:
Each unit is vulnerable to the 2 units to its left and is good against the 2 units to its right. So the first unit Light Melee is good against Light Range/Mage and vulnerable to Heavy Range/Heavy Melee (I look at it as circling back around, if that makes sense). Mages are vulnerable to Light Melee/Light Range (to its left) and good against Heavy Melee/Heavy Range (to its right).
The devil is in the details. Though your Sword Dancer (elves light melee) is 'best against' light range and mage, it may not be the best unit available against them. Archers (elves light range) are also 'best against' mage (and heavy melee) and their movement and strike range can be a huge advantage over the slow moving, close strike Sword Dancers. This can be especially important in an actual fight where the other enemy units are Heavy Melee. Those Sword Dancers will be crushed by the Heavy Melee and are usually in range of their attack once they manage to reach the mages.
When you tap the 'i' for more info on mobile it just lists 'best against' and those 2 unit types, another disadvantage. Browser provides detailed info on specific percent attack and defense bonus against those units as well as health and attack numbers for that unit. It also provides movement and strike range, strike-back capabilities (where present) as well as initiative. Mykan's Guide has a detailed description of each of the units. The Wiki has a chart of that info by unit at:
Unit Stats Overview
Again, on browser I can have any of those resources open in another browser tab and flip back and forth as needed while I'm fighting.
Mobile is definitely at a combat disadvantage in many ways. I hope some of this is helpful.