(They're going to show Master Chief who the manlier men are. Master Chief's full title is Master Chief Petty Officer John-117, but "petty" also means "insignificant", so Leonidas is calling Master Chief inferior when compared to him.)
The biggest mistake that you've ever made!
(Leonidas tells Master Chief that he should not have crossed swords with him.)
I'll toss you like a frag grenade!
(Leonidas will throw him like a fragmentation grenade, a common pickup item in the Halo franchise.)
I'll stomp you in the face with my sandals enraged,
(After he defeats him, Leonidas will stomp on Chief's face with his sandals, since the ancient Greeks didn't have any other footwear.)
And tonight, we shall rhyme in the shade!
(In the film 300, when a Persian scout informs the Spartan army that the Persian army plans to block the sun with arrows, a Spartan scout replies, "Then we will fight in the shade." This means that even if the Persian attacks darken the skies, the Spartans will continue fighting in the resulting shadows. As a result, when Master Chief gives out his raps, Leonidas will just evade them and be able to fight back at him. The first part of the sentence, "Tonight, we shall," comes from another famous quote in the movie, in which Leonidas says, "Tonight, we shall dine in hell!")
Your puny fans are fat nerds on computers,
(Halo players, like many computer/video game fans, don't exercise often and are thus obese to some degree, according to Leonidas.)
Jerking off to games, giving themselves first-person shooters!
(A stereotypical gamer is also seen as a nerdy young man who normally plays first-person shooter games, such as Halo, which shows the game viewed in first-person perspective. Since many of these games feature women with accentuated sexual traits, the gamer may masturbate often and would "shoot" in first-person from their perspective.)
Your armor's hard, but my abs are harder!
(Master Chief wears protective armor, but it doesn't come close to Leonidas' powerful musculature.)
You're in my hood now, Chief! THIS IS SPARTA!
(Master Chief is on Leonidas' turf now, otherwise known as Sparta, and thus Leonidas will do to him what he did to the Persians who visited him. In signature style, he kicks him into a bottomless pit, like he had done to the Persian messenger in the movie "300". During this scene, he also yells his famous line, "This is Sparta!", which is best known by its memes and remixes on YouTube.)
(Master Chief flies back up from the hole he was kicked in, and dramatically tells Leonidas not to dismiss him too quickly.)
Cortana says you're Greek, so why don't you stick these lyrics up your ass?
(Cortana is Chief's companion, an A.I., who tells Chief information about the enemies he faces. To "go Greek" on someone means to have anal sex with them, so Master Chief tells Leonidas to shove his rhymes up his ass. Leonidas was Greek, and Greeks were known to be open about being homosexual, so they would sometimes have anal sex.)
They built a monument to my sins. You're the soldier they need you to be.
(In Halo 2, the Gravemind introduces himself to Master Chief by saying, "I am a monument to all your sins." There is also a monument built to showcase Chief's heroism in the Museum of Humanity in Halo. Additionally, "A Monument To All Your Sins" is an achievement in Halo: Reach, which is earned by completing the campaign on the hardest difficulty known as Legendary. In Halo: Reach, "The Soldier We Need You to Be" is the lowest rank you can get, so Chief calls Leonidas a weak soldier, capable of only the easiest difficulty.)
Ain't no way that you can beat me; even my initials spell MC!
(An MC, or emcee, is another way to say rapper. Master Chief is saying that his initials prove that he's already a better rapper than Leonidas.)
While you and your companions were all camping in a canyon,
(Leonidas took his stand against the Persians in a canyon during the Battle of Thermopylae. Chief is also referring to the common first-person shooter tactic of staying in one spot to achieve more kills, or "camping". It is considered highly annoying to other gamers to do this, as it seems like a cheating way of winning the game.)
Took a campaign to your house and showed your Queen my plasma cannon!
(A reference to the "campaign mode" of many first-person shooters in which multiple missions may be requested of the player to reach a central objective within a defined setting. Here, Chief is referencing 300, when the messenger and his men impeded on his house, and Queen Gorgo slept with Theron for his promise of help. A plasma cannon is also a weapon in the Halo series, so Chief is saying he had sex with Leonidas' wife, "plasma cannon" being the metaphor for his penis.)
They should've thrown your rhymes over the cliff because they're sickly.
(Weak Spartans in ancient Sparta were kicked off a cliff because they were considered to be physically unfit for combat. Leonidas was born with a defect, but his parents chose to let him live, even though it was against the law. Chief is saying Leonidas' raps are so bad, they should suffer the same fate weak Spartans receive at birth.)
You will not enjoy this, but it will be over quickly.
(Master Chief is stating that this will be a painful battle for Leonidas, but Chief will end it mercifully early for him. He is also quoting a line spoken by Theron to Queen Gorgo prior to Leonidas' battle at Thermopylae, in which a he blackmailed the queen into having sex with him, and before the rape commenced he said, "This will not be over quickly. You will not enjoy this. I am not your King." Later in the movie, the queen exposes that the traitor was bribed into betraying Sparta by stabbing him with a borrowed blade and uttering, "This will not be over quickly. You will not enjoy this. I am not your Queen!")
Ha! I've had better battles with my six-year-old son!
(From a young age, Spartans were trained to be warriors. In the film 300, a scene shows Leonidas fighting his son, Pleistarchus, as a form of training. Leonidas is implying that even his six-year-old son is stronger than Master Chief, and that he was more challenged in the training he had with his son rather than the battle he has with Master Chief. This could also be a very subtle reference to John (Master Chief)'s abduction from his home world by the military at the age of six to undergo the SPARTAN project and become the super-soldier he is, as depicted in the first Halo novel. A Spartan boy doesn't become a true Spartan until the age of seven. Leonidas is saying Master Chief is a worse fighter than a small child and can't match to a true Spartan.)
I don't need firepower when I'm rocking these guns!
("Guns" are another term for huge biceps, like the kind Leonidas has. Since the Spartans never had guns, and used their brawn instead, he is saying that he doesn't need the weapons used in Halo to fight his enemies.)
I'm king! You sleep in a freezer in outer space!
(Leonidas was King of Sparta. Master Chief was placed in cryogenic hibernation as the Pillar of Autumn, the spaceship he was on, was fleeing the besieged planet Reach and was under hot pursuit by hostile forces. It could also be a reference to the ending of Halo 3, where the Master Chief enters cryogenic hibernation for several years due to him becoming stranded in space.)
I'd look you in the eyes, but you're too much of a bitch to show your face!
(Master Chief is famously known for never showing his face in the entire Halo franchise. Leonidas is saying that Chief does this because he has no self-confidence.)
You got a bad case of no shirt there, Fabio Flintstone.
(Leonidas wore no shirts, similar to Fabio Lanzoni, a famous Italian model depicted to be often shirtless. Master Chief makes fun of his build by saying he's a caveman version of Fabio, since he came from the B.C. era like the Flintstones.)
Your whole plan got messed up by a hunchback with Down syndrome!
(Leonidas and his men were all killed in the battle of Thermopylae because a hunchbacked traitor named Ephialtes, who appears to be deformed, betrayed the Spartans. Down syndrome is a disorder resulting from an errant extra chromosome inherited by affected individuals that can cause deformity and that may also impair mental capacity, so Master Chief taunts the Spartans by saying that their entire strategy was defeated because of Ephialtes, who should have been thrown over the cliff for having a disability.)
300 asses need a kicking, give more tea bags than Lipton,
(There were 300 Spartans in the Battle of Thermopylae; Master Chief is saying he can defeat every one of them since Spartans in Halo are capable of defeating an entire group of 300 people. "Teabagging" is a term referring to the act of placing a man's genitals on a victim's face and in his/her mouth, which is commonly done in Halo simply by crouching rapidly on a dead body. Lipton is a brand of tea most often sold in bags, so Master Chief says that he'll give more teabags than Lipton produces.)
So why don't you quit your bitching? My trigger finger's itching!
(Master Chief tells Leonidas he should stop talking and trying to rap against him, or he'll shoot him with his trigger finger, which is the act of shooting rapidly with a semi-automatic firearm.)