you say you have problems aiming quickly and on target?
1 my lil bit of advice is make sure that the enemies are as close to the center of your TV screen as possible before you pull the aim button in rather than trying 2 drag the sight across the screen
2 try using slight of hand and get it 2 pro spec this helps you aim down sights faster
3 try changing your button lay out 2 tactical so you can drop shot ppl this also makes you a smaller more difficult target 2 hit (not stood there like a lemon)
4 dont just turn your sensitively straight up to 10 or u will end up running and looking around like stevie wonder ( all over the place ) i find that 7 is good for SMG or AR's 10 is for sniping
5 try completing the story mode on hardened or veteran to get your reaction times down and above all practice practice practice
and please say thanks if ive helped you

as i havent wrote this much since leaving school 10years ago