Welcome, hope you find the game as amazing as I do.
Opposing fire gives the defender the same advantage that it gives him IRL: notably, he gets to shoot first.
That is why you need numbers in order to defeat a defender. One on one and he gets the advantage, but two to one (or even better, 3 to 1 or more) and you basically guarantee that somebody will get a shot off. You also have to remember to use the numerical advantage every single time. Dont just move a unit and constantly try to rally it and fire, triggering OP fire after OP fire on the same target. Move one unit, have it engaged, then another one, then another one etc. Then fire, each time with a different unit. This can be done by just two units i.e. move unit 1, move unit 2, fire with unit 1, fire with unit 2, etc. By doung this, you make sure defender switches target every time, negating "zeroing" effects and dispersing damage (soy you dont have just one unit getting casualties and breaking).
Alternatively (and especially if you have heavy weapons like MGs, AGLs, direct firing artillery or tanks), you use an overwatch element and a maneuver element. Maneuver element moves, triggers fire, detects the enemy and at this point, overwatch fire suppresses him, causes losses, and enables maneuver element to close in and assault.
If you have artillery, bombard any position you think it has enemy troops occupying it. The idea is not to destroy him, but suppress him, reducing the number of shots he gets, reducing accuracy and morale. Then, maneuver elements can do the same as above.
The above work more in infantry assaulting infantry scenario. In a tank vs. infantry scenario, try to stay outside of portable AT range and engage at stand off range. Infantry helps in being the scouts for the tanks.
For tank vs. tank scenario, your options are limited in numbers and overwatch/maneuver. In the second case, the overwatch may be AT weaponry like dismounted ATGMs which are hard to detect, or other tanks. In tanks assaulting ATGMs, rely ojn artillery reducing ATGM effectiveness. In certain cases, you may rely on armor thickness absorbing hits, but it is a risky scenario, since side shots are a thing.
A very brief summary of how to deal with the defender OP fire problem. If there's another scenario I did not remember tell me and I'll write what you need to do.
Hope that helps!
