Other experienced players, feel free to add or correct.
1.) Focus fire. When engaging enemies, pick one and stay on him. A dead Skyhammer before he can throw his airstrike is better than a half-dead Skyhammer and a half-dead Fragger, both of whom can go on to use their abilities. Remember, the most important hitpoint is the last.
1a.) Always pick the one doing the objective. When this is inapplicable, pick the one that can kill you fastest.
2.) Playing objective when non-Engy. Don’t shy away from doing the objective; however, once an Engy arrives on scene, drop back and cover him. It only takes a few minutes before you run across people like me bitching about more than one person accomplishing the objective at a time.
2a.) If a Medic, check for fallen Engies and revive them before going for the objective. Reviving and healing in general should take priority even if all you have is a Fragger to revive. Hopefully he’ll cover you, but it’s better than nothing. It’s faster to revive a Bushwhacker and have him (dis)arm C4 than to disarm it even as your CR73 Sawbonez.
3.) Just because you’re an Engy doesn’t mean you can do the objective faster than you can die. Make sure to protect yourself when doing the objective.
3a.) However, if your death came immediately after a clutch plant/defuse? Worth it.
3b.) Bushwhacker is great for when your team refuses to back you up, but the Turret has less health than you think. You can also reset the Turret’s health by picking it up and reclaiming its cooldown.
4.) Aura and Sawbonez. Charge your damn paddles! To do this, hold down the button before issuing the revive. They crackle when fully charged and completely replenish the target’s health when charged up. Obviously it’s better to get more people up than to piddle around charging up everyone’s revive, but when running up to a group of downed friendlies, charge your paddles on approach so as to revive the first at full health.
4a.) Revive target priorities. Engies first when close to objective. Then other Medics, then everyone else. When in doubt, just revive as many as you can.
4b.) Sawbonez and Sparks can direct-hit with their respective kits for extra heals; prioritize the lowest health players and those on the objective.
4c.) Medics in this game aren’t like most other medics. If you’ve played the Assault class in Battlefield games, you know what Sawbonez in particular is. You’re a Medic, and yes, keeping your friends alive is number one, but get your ass in the fight and do some damage. Helping kill an enemy before they can damage your friends? That’s preventative medicine.
5.) When a Fire Support merc, support your teammates’ fire by keeping those ammo kits coming. Timing is more important than constant bombardment for your air support/artillery strike/Death Star turbolaser.
6.) Shotguns. DO NOT SPAM
7.) Fragger’s LMG and 'nades. DO NOT SPAM
8.) Fletcher’s stickies. DO NOT SPAM
9.) Proxy’s landmines. Place them around blind corners and other pain-in-the-ass locations where shooting them will still leave the enemy in an awkward position.
10.) You have a secondary weapon. Don’t be afraid to use it. Rhinos especially, the pistol makes a great way to deal parting shots.