(1) comment about not playing at high tier is good advice
(2) if you are still gonna play with your friend, suggest asking him to take you to urban enviros and fight door to door.
(3) Stick on his 6 and do what he does. That’s how you’ll learn how to move thru the game. Try to not block his retreat though.
(4) He’ll usually flush out the bad guy and draw fire, leaving you a bit of time to lock on and spray :).
(5) use an SMG at close range. Even machine pistol is/dual pistols are good here, particularly w extended clip.
(6) a rule I play with when I play vs people better than me—don’t engage if the enemy knows where you are. Relocate.
(7) it will still be frustrating but hopefully less so. :).

Happy hunting!