there are short cuts that are meant to be and sum cause bodily harm to get to.

Levels wit shortcutsthat i've found)
1-not any that r important
2-yea rite u wish
3-dont think so
4-u can force the warthog b/t 2 trees (w/o grenades, just aim)
then when u get to the "shafted" cutscene u can jump to sum lower levels(careful u hav to do it jus rite) then the obvious one, jump directly on the over shield.
5- the banshee shortcut, get the banshee fly thru the rest of the level and there r no more enemies.
6-there is but i havent been able to stay on the pelican w/o falling off(watch for elite flood in very begining on the fallen tree very top)
7-library ha i doubt it but if so tell me
8-none really
9-nope but i have made it across the big hole in the floor(with the help of a FRAG grenade)
10-no shortcuts but sum special areas, shoot thru glass area, & the glitch in the engine room to kill all sentinals at once(works better w/ co-op.)

easy or Legendary...hour and a half , yea rite ......BS