How the middle marine forge works:
If a marine (only a marine) enters within a certain distance, and the forge is currently unowned, control is given to the marine owner.
If a mariner leaves a slightly smaller distance, control is reverted to neutral.
Also, every minute, control is reverted to neutral. Presumably to fix up any issues with triggers not firing, which would allow the player to control it for the whole game if they glitched it.
As I said before, if you send TWO marines near the forge, and ONE leaves, then control goes back to neutral. This is because it always goes back to neutral if it detects any marine leaving the magic radius, and it will do so until you send another marine into the radius. If you are sending a large amount of marines, chances are that this will happen just because marines are pushing each other around trying to get to their destination.
The 100% sure fire way to use it is to only send 1 marine right up next to the marine forge. Its still possible that the revert-to-neutral-every-minute thing will hit right when you are using it, so if that happens move your marine back a few inches then to the forge again.
My usual anti-stalker strategy (for non-750 mineral starts) is simply Marauders and Marines. You already have the early game lead because you go with cheaper marauders. You use the marauders to tank for marines with upgraded range. Stimpacks make your army output massively more damage then stalkers, although they aren't as durable for a long time.
The weakness of stalkers is that they are really large units, and their armor/health is split.
While they have range and damage, the fact that they are large units means that for every spot taken up by a stalker on one side, 4 marines or 2 marauders can fit in it. 4 marines or 2 marauders under stimpack do so much more damage then stalkers it isn't even funny. Since you have about 3 units firing to every 1 of his, and since you are on stimpack which makes your units fire about 3x faster then his, every damage upgrade you get is roughly 9x more powerful then his. Which is why I say that stalkers are only a problem early game, as soon as you get to the break point of marine range and about 3/3/3 marine rate of fire/damage/health, you start to totally obliterate stalkers by use of focus fire and the upgrade power. If its a very large battle with relatively open terrain, give your M/M group an order to move FORWARD into the stalker formation and then fire. The reason? Again, for every extra stalker firing because you moved forward, 4 more marines on your side are piling it on. Marines with rate of fire upgrades AND stimpacks AND some damage upgrades will make stalkers melt away, you just want to make sure as many are firing as possible since your marines have the numbers advantaged.
The other problem, health/shield split, means armor and shield upgrades are effectively useless for stalkers in any reasonable game. Why? They are split in half. Damage upgrades would be massively weaker if they only worked half the time and still cost the same amount, its the same story here. On the other hand, the enemy upgrading to stalker lancers actually makes your OWN armor upgrade MORE effective. Massive amount of weaker attacks is what armor is for. Of course, once they start getting lancer damage they take it back since they can get more damage then you can get armor, but rushing lancers without damage can hand you the advantage if you invested in armor.
- Everyone seems to want to go void rays every game and when they do, we know we've won. It costs 2000 for the building yet upgraded marines waste them every time.
Void rays aren't intended for fighting armies, they are intended to snipe buildings. 5/10 of them snipe your merc compound. 40 of them snipe your CC and its game over. I assume people who are going void rays, though, simply see that it costs the most and assume its the best overall. lulz.
That said, if you really want to screw over flying mercs, invest in the infestor. Plague will immobilize the entire group while your marines sit there shooting at them outside their range, with air units unable to shoot back. HILARIOUS.