Early reinforcements at the evac zone
Posted January 3rd 2012
As you'll doubtless already know, the evac zone battle can take place in either of two locations. When you get sufficiently far down one side of the valley, the game selects the furthest area as the evac zone, and a high dropship moves to a preliminary hovering position closer to it, bombarding three troopers. As you approach the zone, the ship heads in to drop seven Grunts. The troopers are actually invulnerable to begin with, only normalizing when you've got closer after officially locating them (blue indicators appearing).
In normal play it often seems like you have to kill most or all of the Grunts before the two reinforcement ships are triggered (the exact workings are unclear to me). But it's possible to make things work differently. If you arrive sufficiently far ahead of that first ship, the reinforcements get triggered pretty much immediately, with ship 2 maybe spawning even before the first ship has started opening its doors. As such, you're potentially in for a more intense battle, maybe with more enemies on the ground at once.
Arriving sufficiently early at the lower zone
To arrive sufficiently early at the lower evac zone, it's best to be in Jun's Falcon and take a shortcut route around the right as follows. At top speed, curve left around a pinnacle just past the start of the broken bridge so you end up passing over the circular structure. Pick your way safely through or over the treetops and cruise in low towards the troopers. They can get located a second or two before the dropship has reached its preliminary hovering position, where it pauses momentarily before continuing on to make the drop.
The only other arrival method I had success with uses a truck. First head around the right of the valley far enough to trigger the dropship to head nearer the lower evac zone, then about-face and go around to approach the zone from the left. The final approach should be from behind at top speed. Instead of taking the standard path which ends up curving rightwards towards the troopers, take a straight-line shortcut so you end up jumping the truck off the raised area behind the curved tin shelter and landing near the troopers (an enjoyable jump, by the way). There's a decent chance of success - around 50% when I tested.
Aside: One time when I arrived at speed from behind like that, the truck hit a trooper who then deflected up off some wreckage and flew across the map in a high arc, landing and dying up on a distant clifftop area behind the buildings. Impressive!
Arriving sufficiently early at the higher zone
To arrive sufficiently early at the higher evac zone, just approach along the right in a truck or Jun's Falcon, after going far enough along the left of the valley to make the game select that zone. The dropship's preliminary hovering position is actually quite far off - well beyond the zone as you approach from the right - and you've got plenty of time to beat it to the troopers. Ship 2 can be spawning even before the first ship has settled into position for its drop, though you won't actually see it for a few seconds because the spawn point is around a corner. You tend to see it while Grunts are dismounting.
It's also possible to arrive early enough by flying in from behind (more or less) after cruising through treetops. It's an awkward flight, but it would save you having to turn back to approach from the other way.
Carter and Jorge
If you're arriving via Falcon and you want Carter and Jorge to play their normal part in the battle, don't have them aboard, because they'll just stay with the Falcon, not dismounting at all unless you get sufficiently far away, in which case Carter is likely to take the pilot's seat and clumsily attempt to fly off. If you arrive alone they'll probably both materialize there, unless perhaps Carter took the truck at rally point alpha - something I think you can forestall by briefly driving it solo to the Falcon after bringing it down.
What's going on?
Here's a possible explanation for the unusually early reinforcements. Once you've located the troopers, the game surely monitors how many dropship troops are on the scene, as part of deciding when to trigger the reinforcement ships. The exact conditions for the triggering are unclear to me, resisting my attempts at investigation, but one thing I'm sure of is that if the count is zero, that would suffice. When you arrive unexpectedly early as described here, I suspect the game decides there are no dropship troops on the scene - either because of the unusual remoteness of the first ship or because no troops have spawned inside it yet (I'm not sure when they spawn) - and hence the reinforcements are triggered right away.