To level up your characters you need to focus on each one.
The medic Has low range but high rps weapons and high range low rps weapons. I think you should start with him.
The assassin is for the ones who have a pretty good skill on switching between weapons very fastly, also you need to know how to headshot and do this in movement. Actually i think that he is the best for capture the flag, because while the tank of the team is going to get the flag, he is protecting the tank like a "scout" with his sniper. Also, despite the fact of low hp, in level 30 he is in about 110 HP, and that's not bad.
The Commando is similar to medic's low range and high rps weapons, and also very low range weapons, missile launchers.
The tank is a good blocker for protecting all of the other team mates.
I suggest a set of skills/killstreaks:
For medic - Self-Revive and Kevlar Vests.
For assassin - Blur and Surge.
For commando - Martyrdom and Bouncing Betty.
For tank - Iron Will and Ricochet.