Overwatch is a game which relies on coordination to initiate success. Target prioritization plays a huge part in this. Unfortunately, many players do not understand the basics of prioritization when all it really takes is a brief moment of evaluating the scenario.
For example, the enemy is running a comp on defense that is not too great. Their DPS are Genji, Mei, and Hanzo. Their tanks are Dva and Roadhog. Their support is Lucio. Your team is running a standard 2-2-2: Rein, Zarya, McCree, Reaper, Lucio, Ana. For some reason, they are actually beating you despite the fact that they have an inferior comp.
What should be done here? It should be apparent that they running only one healer. Lucio needs to be killed, ensuring that the team cannot get the AoE heals he provides. Without the solo healer dead, the DPS generated from your team is simply trash damage and is only used to charge the Lucio's ult.
That was a simplistic example. This time the enemy is running a Rein, Zarya, McCree, Mei, Lucio, Mercy. Now do not be deceived by what I wrote before. In this scenario it is actually vital for McCree to be targeted. Why? Take a look at their DPS. Mercy and Lucio are both main healers, so hardly any damage is generated from them. Mei does not provide enough damage on her own like McCree does. After McCree is dealt with, your team can progress to a healer and a tank, then eliminating the rest.
Now to make it more complex. The enemy is running a Winston, Zarya, Genji, Reaper, Lucio, and Zen on attack. This is a pretty good dive comp. Throughout the game, your back line is harassed by the Winston and the Genji. Sure, this problem might be because of team composition, but let's say that this time it is not. Before, we looked at who is most important in the team's composition. Now you must look at the team's enablers. Winston and Genji are not entirely powerful on their own like McCree is. Who synergizes with them in order to make them powerful? Their Zenyatta enables Winston's low damage fan attack to become a devastating force of nature for your squishies. Those shurikens that only poked you now deal a good amount of damage. This is where your target prioritization might needs to be split. If you allow your back line to be harassed by the divers, you will have no luck in killing zen. If you simply focus on the divers, Zarya will be charged up, and they will have extra damage to tear right through you. Winston and Genji need to be kept at bay while you find a way to focus Zen. This can be done by CC (Crowd Control) or coordination your defense and defensive attack. Remember, Zarya can only have a bubble on one other hero and herself at a time. If Zen is harassed while their Divers are attacking, Zarya can only help one.
Again, all of this is about quick thinking and understanding what the other team is doing to counter yours. Mentally breaking down their team comp and your strategy will lead to success in the game. As an added note which I felt not to delve into in this thread is the idea of knowing who has ultimates on the enemy team. A hard push can be instantly stopped by a Mei's Blizzard even if half their team is dead.
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