I would recommend starting with an iron clad foundation for any team. Take the healer and tank roles, and master them. I’d recommend either ISIC or Montana for the Tank, and Miko, Alani or Kleese for the heals. All five are versatile characters who can stuff the early shots of the enemy team. A terrible tactic I’m seeing, is that a lot of people are picking small fast DPS, thinking that they can overwhelm or swarm an enemy team with too much DPS to counter. And as long as they have no tank or heals, it might work.
To be clear, this isn’t an issue of skill level, but encouragement. A lot of lightweight DPS players, will CONSTANTLY hide, waiting for someone else to over-extend and draw out the enemy to pounce on a kill. Again, it can work, but it generally results in at best a wash(they get a kill, your team gets a kill) or they come out ahead(they get one kill and escape, or they get one kill, and then kill the pouncer for a second) But a solid tank that can stay out there and draw out the enemy, and a healer that can keep the tank up, can draw out a fight long enough for the pouncer to make the kill, and the tank and healer to get assists, which helps immensely. With that tank able to stuff those early shots, the DPS will roam a little more, and be in better positions for ganks, instead of trying to stay out of sight or out of shot-line.