XP earning difficulties IMO
Medic(Easy) - Healing your mate earns you way more XP than sucking up ammo with the phoenix. Look for the location of your group, the closer the group stick together, the more XP you're likely to get if you aim at the groundin the middle.
Spec Ops(Normal) - Give away all the ammo, keep the soldier barrier in check, remember no soldier means nobody to take the ammo, Scope and HS everything inside while waiting for the ammo to replenish. Midic can generate their own phoenix ammo, so only give them ammo if you have extra or waiting to approach the next objective. The sparkball nade replenish everytime you throw an ammo pack so spamming it for extra XP.
Soldier(Hard) - Barrier is your friend, keep looking around to make sure nobody can fire at you from the side or back. The closer to the target, the more XP you're gonna get. Keep granade for the best oppotunity. Run for your life if you are out of the barrier :lol .
Anyway, I've been playing so many random maps. I never actually having problem with Medic, they seems to know what they are doing. Selfish Spec Ops is the problem, they will be sitting far away sniping and throwing the ammo pack to the groun just for give themselves ammo. Such SO is a Soldier nightmares, no chance of winning without ammo.
The best team porpotion I found is 3 Soldiers 3 Spec Ops 2 Medics(Or 3 Medics and 2 SOs).
I love playing medic because they dont have to rely on anyone, and support role in this game is kinda rewarding as well.