Always take opportunity to seek and kill a player who is alone.
Try to intercept player via different route, rather than chaising him though the map.
DMR is good in close-mid-long ranges.
Pistol>Dmr fire rate.
DMR>Pistol precision.
Use walls to bounce of granades rather than chuck them right under enemy feet(frag granades).
Evade should be used to evade, not to outrun enemy.
If you got the sniper rifle, try to hold it right at heads height, so in case you run into enemy, you can get insta no-scope.
Don't shoot rocket launcher directly to enemy player, if has splash damage equal to atomic bomb, shoot it at his/her feet instead.
If someones shields are down, go for head shoot.
There is no use running away from show player who has sprint.(try to kill him instead).
Just some of my basic rules( other players have different ones and differ from my opinion).