Campaign

Shoot and hide. Also,take out the snipers.

Don't go as fast through the level and try to kill everything from a distance

Take out the snipers and the grunts first, they're the most annoying. Then basically I just brawl.. But you probably don't have the same technique as I do.

As everyone else said, snipers are a must to take out. In higher difficulties there are more Jackle snipers and sometimes even Brute snipers. Also choose your weapons wisely, don't always go for short range weapons in higher difficulties, choose Sniper Rifle or Battle Rifle over the Assult Rifle for example. Never be afraid to take your time on just one section of a mission, it's much better than getting killed over and over again because of poor estimations of how powerful your enemies are. Sorry for such a long answer it's just me.
Hope I helped though

You should get a freind to play with you in parts that are hard one of you stay back and the other one kill things then when you die you respawn and you go again it really works.

Go through at a steady pace, don't rush.
Also aim for the head. With a Battle rifle it can take like 7 - 8 bursts to kill a grunt but only one burst to the head for a kill.

Shoot and hide, or try sniping.

if u have grunts birthday party put it on because I've had the confetti explosion kill other grunts and it also spers u on to get head shots. NEVER try 2 hit someone. shoot an and run. the arbiter can do some damage if u stay back. co-op. don't use the assault rifle. go slowly. carry a cloaking device for cheaftens expesaly the one on level 6. grunts are very annoying use a grenade if possible. a good weapon combination is a plasma pistol dueled with a magnum plasma for shields following with a head shot. soz for the long answer hope i helped. my gamer tag is LazyData if u need help