First off, Make sure you understand two key things of Positioning and a hero’s strengths.
Resources can be found here:
The common assumption:
“Because a hero was good on Attack ill just play them on Defence.”
The problem is you can play a attack hero in a playstyle that is defensive but it does not work to the heroes strengths.
An Attack hero is best suited for attack and a defence hero is good for Defence.
BUT and its a ~THICC but.
If you intent to play an Attack hero on Defence you must actively Flank the enemy to be effective.
And if your playing a Defence hero on Attack you must actively peel against enemy flankers, improve the teams sustain Or flank depending on the hero.
Composition:
To make a good composition you but first consider the group of heroes suited to the round your about to play as mentioned in the hero pools review.
They suggest those groups of heroes to your teammates for a stronger team.
Compositional balance:
Once your team has a group of defensive or Offensive heroes you can look at the heroes in your team and choose a better suited hero that can assist the team better.
An example:
If you have a Winston you can choose Ashe to throw dynamite where Winston dives and stack the damage with his.
Compositional Counters:
Now this requires experience with Positioning and the types of heroes used for what purpose. Its not always what single hero can counter the whole team its what hero can counter the position the enemy is in.
On Attack:
If you were to face a enemy team on Defence with Backline heavy comp you can choose Midline heroes to enfoce the Frontline tanks. The counter is when you have more pressure on the Frontline than the enemy they fall back because their tanks cannot sustain their Frontline for as long because of the sheer DPS of the team.
If the enemy had a Midline heavy comp on Defence then Flankline heroes would be the counter. As a Flankline Hero, it is your goal to split the attention of the enemy off your teams Frontline.
If the enemy had a Flankline heavy comp then choosing Defensive heroes will counter this.
On Defence:
If the enemy had a heavy Midline comp on attack then Flankline heroes are the counter and the Frontline and Midline heroes must use the terrain as cover as much as possible.
If the enemy had a heavy Backline comp on attack then you can counter with a Midline heavy comp with Defensive support to avoid being picked off by the enemy Backline.
If the enemy had a heavy Flankline comp on Attack then defensive heroes are the counter.
Where it gets complicated:
Since most teams consist of 2-2-2 and are sometimes a mix of Positional DPS like one on the Backline and one on the Flankline.
In this instance they have weakened their team composition.
You can choose one or two heroes that counters them and focus them, you must do so quickly in a way that does prevents them for achieveing their goal.
You will probably see many wierd comps the enemy comes up with but ultimately, if your team has a balanced composition then it will be difficult to counter you without proper counter compositions.
Think of it like this; they send 1 Flankline enemy to counter your Midline comp, That is technically a 2v1 right there unless they outplay you but if you have Midline Support then is 3v1.
Assuming to read the other guides, practice this in QP till you get a feel for it and then the rest is childs play.
Thanks for reading.