The Metal Gears aren't too hard when you have the knack.
1. You are wearing the body armour aren't you?
2. Make sure chaff is in the air at all times. (the missiles are then less accurate)
3. Don't forget stingers are fire and forget: get lock, fire, switch back to dodging, rinse, and repeat.
4. shoot the metal gear in the knee with a stinger to get it to bend down, then follow up with one in the open mouth bit for loads of damage. 3/4 head hits per metal gear.
More MG's come in (you have to kill 5-9 total depending on what skill you are playing on I believe) and when you ahve done enough damage more tedious cutscenes kick in (it's not a bad one actually)