The Strats Edit
As the level starts run towards the river and use the rock to jump on a rock ledge and grenade jump to the cliff above (1/5). This skips the first enemy spawn up ahead and saves a few seconds.
It's also viable to run across the bridge, especially if you are inconsistent with grenade jumps or just starting out.
This grenade jump is much more important due to the skipped spawn.
Once you reach the base, assist the marines with killing the first drop ship spawn. Once the enemies are dead, kill all of the marines and proceed to the next drop ship on the other side of the base. Killing the marines skips dialogue and means you don't have to wait for them to evac.
You should probably save frags for the 3rd dropship to kill the elite, and the 5th dropship for the ease of killing the spawn. Once you have killed all dropship spawns grab the warthog and get the marine out of the turret and then continue along the intended route.
The Light BridgeEdit
At the bridge segment, use the warthog to jump on the slanted wall heading up to the console, this will skip a cutscene and instantly start the bridge. Proceed out of the natural caves and stop by the escape pod on your way to the left most spawn to pick up a sniper rifle for later.
Before getting into the warthog it is recommended to grab a needler. When you go to activate the light bridge throw 2 plasmas down the intended hallway behind you and fire the needler to kill the two elites that are added on legendary.
Through the next areas, you will be killing marines. Be careful about causing the marines to go red - if this happens the level will complete more slowly than normally.
Once you reach the first marine spawn, go inside and kill the marines, proceed towards the next spawn located at the cliffs.
Before heading into the rocks, pick off the sniper located above the marines. Finish off the remaining marine spawn and head towards the last group.
Instead of killing all the marines, drive around the right side in the warthog and reach the top where the marines are holding off. Kill all but the sniper and one other, and allow them to get into the warthog, you will use them at the next base as an efficient way to kill the enemy spawns.
Before entering their area, pause on the hill for the prompt to search for remaining life boats to save time. Drive up to the marines to trigger the area, then proceed to kill the enemy spawn and the dropship that comes afterwards.