Here is a few tips that get me to win.
Don't carry 2 melee/close range weapons at the same time.
once you've got the guys' shields down, get your DMR and get a head shot.
Grenades can be usefull, alot of people say conserve them, but don't hold back. If you're having a jetpack batttle, the best option is to put a plasma grenade on them. SO FRICKEN USEFUL!
Also, if someone is coming at you with an energy sword, melee at the same time they lunge at you, it will knock them back. ( This only works in Reach and 3, not halo 4, although it still works if you have a sword too.
With teams, try using a hologram on someone whos' name is part of their real life name, they are most likely noobs (not all the time though), they will shoot the hologram and you can get a nice clean assassination.
If you are in the middle of a melee battle, watch out, there are usually always others watching you, so keep to the wall/tree/mountain/you get the point.
If someone chucks a plasma on you, just sprint up to them in the hope that they will get killed also, or even damaged, so your teammates can finish him off.
Hope this helps!
p.s. i have crap internet and can't use youtube, sorry.