The Felix Felicis potion, also known as "Liquid luck."
http://harrypotter.wikia.com/wiki/Felix_Felicis
Felix Felicis is a potion that is "Liquid Luck". Felix Felicis makes the drinker lucky for a period of time, depending on how much is taken, during which time everything they attempt will be successful. It must be used sparingly, however, because if taken in excess it causes giddiness, recklessness and dangerous overconfidence. It is highly toxic in large quantities, and is also a banned substance in all organised competitions such as Quidditch, along with all other methods of cheating. It is very difficult to make, disastrous to get wrong and must stew for six months before it is ready to be consumed.
Harry wins a vial of Felix Felicis from Slughorn in potions class after making a perfect concoction of Draught of living death. Enough for "One perfect day." Slughorn literally says "One sip and you'll find that all your endeavors succeed." and he also said that he took it twice in his life, and hd two perfect days.
Now....
Snape is potions master. He simply brews a big ass vat of Felix Felicis, enough for all the wizards to consume, and abra cadabra, they will all succeed in their every endeavor. Including taking the Jedi temple.
NOW.....
The potion takes six months to brew, according to the link I provided. Snape uses the time turner to go back six months prior, leaves himself a note to brew the potion (Or tells Slughorn), and on the day of the attack, presto, liquid luck.
The wizards will have infinite luck, they will succeed at everything they try. A Jedi swings a lightsaber at them, the wizard somehow avoids it, and spells them. No matter what the Jedi throw at them, the wizards will win in the end.
How ya like dem apples?