Joker gives some good advice that I’d follow.
Being good at PVP in any game starts with knowledge - knowledge about the fight mechanics (i.e. energy > bio > mech > energy), knowledge about each hero’s abilities, knowledge about synergies and weaknesses. It’s not easy but if you really want to be good you will devote yourself to this first.
The second step is knowing how to put together a good squad. There are certain combinations that work well together and certain heroes that are universally useful (like gold Mandrake). When I make a new squad, I typically start with a hero (or two) who has a particular strength I want to exploit, then surround him or her with other heroes that will greatly augment that strength and/or offset that hero’s weakness. Ideally both.
For example, Dogface can be extremely devastating in PVP. If he is able to get off both his bronze and silver abilities, he can single-handedly take out 2 or 3 enemy heroes (possibly more). However, he has 2 primary weaknesses: 1) it takes a long time to charge up his abilities and 2) his abilities have a long cast time (they can be interrupted easily). So a good supporting cast for Dogface should include heroes that help him survive long enough to use his abilities (Mandrake, Phalanx, Heimlock, other healers) and are able to disable or distract heroes who can interrupt him (other strong DPS and stunners). This is just one example.
Third, you need to develop the ability to come up with a strategy for dealing with the enemy team quickly. As soon as the loading screen pops up and you see the enemy team composition, you need to be able to come up with a rough kill order, tactics for dealing with particular enemy abilities and determine how you want to control your own team. You will have maybe 5 to 10 seconds to decide all this. Of course you must also be able to adjust your strategy on the fly when things don’t go according to your original plan.
For example, the squad I’ve been using for this current event consists of Hideo, Razorback, Mandrake, Heimlock and Sapphyr. My approach with this team is basically to win by attrition with Razorback while using Hideo for spot burst DPS and to prevent enemies from using key abilities using his stun bomb. Let’s say I encounter the following team: Gammond, Dogface, Mandrake, Sapphyr, Clyde
Based on what I said earlier, some of you may think Dogface should be taken down first. While you may get lucky with this approach, generally I find this to be a mistake for 2 reasons. While Dogface is not a tank, he has a lot of hit points relative to other dps. On top of that, he will go invisible for 8 seconds once he hits 50% thanks to Mandrake’s gold ability. So there is a high probability that, not only will you NOT kill Dogface quickly, but you may allow him to use his bronze and silver abilities while invisible, making him IMMUNE to interruption. This is a death sentence. If I had a strong Odachi on my team, then I could probably kill Dogface quick enough, but this would still be the wrong play imo.
In fact I know from experience that Clyde will do the most damage to my team. If I ignore him for the period of time it takes to kill another hero like Mandrake, there is a good chance that 2 or more of my own heroes will be dead by that time. He also has very low hit points. Maybe the lowest in the game. On top of that, he is the only hero on the enemy team that can stun. Clyde should be my first target. Unfortunately I don’t have any strong biochem DPS on my team so I’ll use my strongest Mech DPS, Hideo. If Clyde targets my Sapphyr first, she might be dead by the time I can kill him, but I consider that an acceptable loss as she is the weakest member of my team and not crucial for success. So rather than switching to Sapphyr to evade, I will maintain focus on Clyde with my Hideo. After Clyde goes down, I will probably move to Sapphyr (next lowest hit points) or Gammond while keeping a close eye on Dogface. As soon as I see Dogface casting I will stun him with Hideo. I typically stay on Hideo because I don’t want to him to use his stun on anyone other than Dogface at any time other than when he is casting. This has happened to me before and cost me matches against Dogface teams. In this scenario the only times I will switch to another hero other than Hideo are 1) well before his stun is charged, and 2) just after he has used his stun. As long as I can keep interrupting Dogface while finishing off other heroes, this should be an easy win.
Finally, you need to be able to execute your strategy effectively. Nothing complicated or special here. Practice makes perfect.
TLDR: Study game mechanics and heroes, develop the ability to come up with a quick strategy and practice makes perfect.