This team is Constructed mainly around team fights (obviously). The reason for all of the AD is to suspect that the opposing team will start building armor like thorn mail, Hourglass and what not. Since they'll be building more armor, that leaves an advantage for Brand.
Since Brand is AP (once again, obviously) his spells will be able to produce more damage. Brand has a lot of moves that do damage to multiple targets, so that leaves straight AP damage for each enemy to take.
Now that each enemy has been getting AP damage from brand and Mundo's Burning Agony, they'll already be at a not so great health. Since Alistar has his knock-up (which is an area spell) He'll be able to stun a good amount of champs if everyone is moshing at mid as expected or something relevant to that, Pantheon can use his grand sky fall (WARNING: PANTHEON SHOULD NOT INITIATE THE TEAM FIGHT FOR THIS REASON). If Pantheon jumps, and Alistar uses his knock up perfectly, the enemy team can take up to a very good amount of damage. Plus his Jump/Stun will allow the team to aim for a more squishy champ.
And now you are curious of what Mundo and Tristana will do to help team fights, or are you? Mundo's cleaver is a great slow, only if minions are cleared (that cleaver has A LOT of damage). Then Tristana will be there to ult some champs closer to the team fights and dish out A LOT of damage (Just a good AD Carry).

Brand as a mid is very helpful since he has pretty decent movement speed with boots purchased, VERY good range on his Pillar, and since you have clarity and mana potions, going back wont be a problem for a good while.
Brand has a pretty simple spell combo or pillar, sear (only if you have the opportunity to land it), Conflagration. If in a team fight, or there are only 2 minions and he's close, us Pyroclasm.
And also Brand is a GREAT farmer.

WARNING: BRAND SHOULD NEVER BE CLOSE TO ANOTHER CHAMP AT ALL TIMES, OPPONENT CHAMP SHOULD BE AT EDGE OF SCREEN AT JUST ABOUT ALL TIMES

Pantheon is a very decent top. He has movement speed, a stun, attack speed (off of runes and mastery ;D), and pretty good at farming.
With Pantheon you want to stay back and keep the lane mediated at mid until you can obtain your Frozen Mallet. Your frozen mallet will give you the HP you need to take some turret and opposing champ hits. But be very cautious of opposing Solo Top's with stuns (Like Riven). Since if they catch you at the turret with almost no minions, you're screwed (obviously).

WARNING: PANTHEON'S ULT SHOULD BE USED WHEN ENEMY TEAM IS ALREADY FIGHTING TURRET OR YOUR TEAM

Since Mundo's cleaver and Burning agony already do a good amount of damage, he should be built all tanky and INDESTRUCTIBLE!!!!!!!!!
Mundo's Jungle path should be
-RED
-WRAITHS
-GOLEMS
-WOLVES
-GIVE BRAND BLUE

After Level 3, since you have your cleaver, ganking will be available. To gank, make sure that your cleaver lands a hit, then Tristana can jump on them, and Alistar can land his knock-up and headbutt the target back for the kill.
Mundo is a good counter jungle too, so when you have a good amount of health INVADE!

WARNING: MUNDO IS SLOW AT THE BEGINNING, BE CAREFUL OF GANKING/ COUNTER-JUNGLING

Alistar should tank A LOT for Tristana.
If Alistar can get Tristana kills, game over, that simple.
Alistar's combo is the knock-up to the headbutt (obviously)
Nothing too much to the bot lane other than keeping the lane mediated and feeding Tristana.

WARNING; ALISTAR ISN'T TANK GOD, SO DON'T TANK TURRETS OR CHAMPS AT THE BEGINNING OF THE MATCH

At the beginning of the match, if you end up dealing a very good amount of damage on an enemy champ, use your ignite and explosive shot and just keep shooting. You'll kill them, or make them leave their lane for a good while.

I bet you're wondering about why there are a lot of Gold building runes. Well it helps A LOT!!!!!!!! In no time you'll be able to buy a B.F. Sword, I promise you, if you don't like it, feel free to use your own page.

WARNING: TRISTANA IS SQUISHY AS FLUFF!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!