Where are you guys getting this "50's are unstoppable" thing? They're no better than any other player, it just means they played for a very long time.
The only time a 50 is unstoppable is when he's raping your spawn from his helicopter on Arica Harbor.
Couple of pro tips:
Use the ABAKAN (2-round burst AK), it's the best gun in the game, IMO, and the only thing that comes close in terms of being overpowered is the M60. You can't have the noobtube on the M60, though.
Chose a sniper class, equip the VSS + red dot (it becomes an incredibly powerful assault rifle) and have the C4 equipped. This is probably the most versatile class option and works on all game modes. Keep throwing those motion sensors out - it helps your team out TREMENDOUSLY. It should just be a habit to always have one out there beeping. Make sure you have a buddy supplying you with ammo because your VSS will run dry really quick.
Blowing up MCOMMs with C4 instead of hitting the X and starting the alarm is by far the best way to generate points in this game. Be smart about it, though, because sneaking to one isn't always easy. If you bumrush it (like I did because I wanted the insignia) you will get killed. A LOT.
Also, you can be an engineer and sneak to the MCOMMs and put mines all around it. They do not disappear after you die so you can keep stacking them. You can shoot or grenade the mines whenever you feel like it for massive MCOMM damage (this pro tip was stolen directly from Kraus, BTW).
If you care about your KDR, Rush is probably not the best game mode, I died way too many times trying to blow those MCOMMs up. It is probably the most satisfying game mode, though.
If you already knew all this, my apologies.