If you run into confrontations you will die

)
Co-op is much easier, as up to 4 Spartans can be firing @ one target, @ any one time.
Hang back & choose your targets from cover.. or keep moving so as they can't land consecutive hits.
& as for the poster who claims they can finish H3 in 4 hours, I say you're telling us of your dreams, not the 360 version

)
Activate a 3-4 of the skulls & play on the normal setting on meta.
This is enough of a varied challenge + enough points can be scored to open the achievements.
Enjoy

)