As a design, Attacking army gets placed near left hand thumb finger and Opponents army gets placed near right hand thumb finger from the army management dialog in mobile. Feeling bit difficult to drag the units using the mobile since need to stretch the finger. Cannot use the deselect/select option per unit popup menu since it gives big trouble because of fat fingers/thumb.
To handle it, suggesting an option in the mobile to swap/toggle the sides of attacking army and opponents army sections in the army management dialog. With this way, right hand thumb easily drag the attacking army units to the right side slots. Also if our defending army gets filled in the army units selection bar, correctly remaining units comes closer to right hand thumb finger to drag or select.
One drawback is having less than 3 units means need to stretch again. IMO, it can be acceptable since under any age every users will have more army units. So no need to worry about it.