I can give some more "deep to the bone" tips for Halo (This is multiplayer based but can be applied against single player AI). I recommend using the DMR and aim for the head
constantly. Even though 4 shots take out a shield then 1 shot to the head gets a kill you'll probably end up becoming sloppy if you don't get use to aiming for the head from the start (This is for practising purposes only, if not practising then do 4 shots body then just make the 5th a headshot). As for secondary weapons magnum or energy pistol work well. The magnum is used for back-up in case you run your clip in the DMR, or can be used as a superior close range weapon to the DMR (Talking about shotgun range or so and rapidly firing for the body/head final shot). The energy pistol can be good for taking out vehicles (charge shot EMP, recommend for any vehicle heavy maps) and as a quick way to take out shields in close range (Takes 4 standard shots I believe) then you can switch to DMR and get the headshot or melee (Results in faster kill than just using DMR since the energy pistols fire rate is close to unrestricted).
As for control scheme a lot of people I know who are good at Halo use bumper jumper so they can jump and aim/shoot at the same time. Unless you practise gaining a fluent claw grip (index finger on face buttons) I recommend this set-up. If you want to practise getting a claw grip just train your ring fingers to pull the triggers, once you can do that begin using your index for the buttons (I use the side of my index) and keep your thumb on the analogue for aiming. Training both left and right hands will help in all shooters as well as all game genres, especially DMC. (Don't give up if you can't do it right away, this is a somewhat rare skill to have and does take a long time to do perfectly)
Also personal preference for me is aim assist off across all FPS, if a game has the option to turn it off I recommend doing it so you don't rely on it. Doing this will make you better overall with practice. However if it's too difficult keep it on until you feel you can't aim any better then take it off. Also once you get use to no aim assist you gain the advantage of being able to do a full spin without having it slow down your speed and making you mess up your aim when an unexpected enemy is nearby, enabling you to pick specific targets easily. As a bonus people that use aim assist can actually be effected by Halo 4's hologram (At least I think, didn't play enough to analyse fully) so you can not only use it as a distraction but can also be used as a way to gain a small but useful advantage against anyone with aim assist on.
P.S This is all from my own experience from what I think is the most versatile set-up possible from a low rank. Use these methods, but if you find after a long time of using them you don't like the style of play go back to what you find most effective for you. If this ends up being the case feel free to ask for tips more suited to your play style.
Also if you need any extra tips for any kind of shooter I have tons more. Rainbow Six, CoD, Battlefield, Mass Effect, Borderlands, you name it. I got tips for almost of 'em specific to how they work and what play styles there are.
Phew that was long, I always have a lot of things to say for games

lol