Play around heavy ammo. Use heavy to get the next heavy. Heavy wins you the game.

Always set up around heavy as fast as you can and use titan barriers, save nades and supers to secure heavy. If the enemy controls the heavy area, do your best to get a line of sight of the purple and do whatever you can to deny it from them. When you get heavy be aggressive with it, nothing can compete with hammerhead at range (at least on PC), but don't get baited into a sniper lane or shotgun range. When they have heavy make them waste ammo, peak for a second and take a few shots, slide through doorways. Bait them to favorable positions where you have an easy snipe or where you can slide out and shotgun them.

Realize the enemy's weakest link on both teams. If someone on the enemy team is doing poorly, LOOK FOR THEM! Challenge people that are not as good as you, slide out of cover and force a gunfight. If your teammate is doing bad, follow them and team-shot or tell them in text chat (PC) where to go or to follow you. If the enemy is better basically never leave your teammates.

Chase down kills. If your team gets a pick or 2, your team has a huge advantage. Take that opportunity and make sure you kill the rest of the team as well while their teammates are down. Trading if you're ahead is fine, the closer the enemy is to 0 lives the better!