Pick a class that works for you and stick with it. I personally used the Bounty Hunter because of the ridiculous cheese factor involved. Many other people prefer Assault or Sapper, so just do what you're most comfortable with.
If you're looking to level up, Zone Control and VIP offer the most opportunities for XP. Spend time there.
Since boosting the game is now quite difficult, I recommend going for the class achievements early on. While you're still in Bronze/Silver tier, you'll be up against a lot of people playing on phones with awful connections. If you have a controller and decent internet, you should be able to destroy them. Use this to your advantage, pick classes with achievements you need and try to go for your killstreaks before being matched up with people who spend every waking moment on the game. Unfortunately if you're playing on the phone yourself, it's unlikely you'll be able to turn and shoot as quickly as someone who is using a controller on a laptop. Odds are you'll be dying a ton after getting into the Gold tier.
Save your SP! You're going to need to have certain skills in certain classes for achievements, specifically the Assault class. Don't throw away your SP early on, because it becomes very hard to obtain past level 50.
If you're serious about the completion, set aside a "Modern Combat fund". $200-$300 (or the equivalent in your local currency) that you can spend as soon as the means to complete the game become available.
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