This dude completely destroyed my team. Please explain how?

Demons and brawlers, being what they had are rather strong right now. Also it depends somewhat on the items you both had, as we can't see what those were, not knowing that info makes it a bit hard.

You have barely any synergys and also your best/3 Star Units are T1 Units.

Brawlers are BROKEN AS FUCK. All you need is 6 brawlers to be huge broken meat tanks and 2-3 carry champs behind them to win. No matter what you go with 6 brawlers you can't lose and its a fact at this point. 6 Brawlers 3 sorcs, 2 gunslingers + random, Demons, Elementals, Kayle + Sej + what ever even Wild and 3 assassins can do the job.. it doesn't matter. they beat Glacials Demons Hextech probobly only lose to Gunslingers or Blademasters that have the best luck in the world and keep procing extra attacks non stop.. BS comp and everyone who runs brawlers needs to feels like an abuser... At best 6 nobles can beat them but good luck getting 6 nobles every game its x100 times more easy to get 6 brawlers every game because they don't depends on a legendary unit.

Btw off topic don't go for level 3 units its pointless. Go for level 8-9 and synergies. focus on level 2ing champs not level 1 or 3.

and on a 2n side note Brawlers with void and sorc is busted. True dmg from hydra on warmog void brawler or ionic spark stacking is dumb because all that dmg does gets converted to true.. Swapping sorcs until you 3 item a kart is dumb as well. On top of that they are flexable with Spac items.. Brawlers needs a huge nerf.

Imho 6 brawlers + kass + kart + Lulu = best team comp.

Okay since no one's outright answering:

The reason his brawlers beat your Knights/Mobles/Draven etc..... isn't because he's Brawler.

It's because :

1. He could probably pull your Draven every fight, removing your one source of long range damage.
2. His comp had 4 sources of back row damage to delete not only the front line, but the back at the same time. Comps that do massive aoes win more than tank every time.
3. Morg/Brand have the new Demon, and Demon was buffed hard to generate mana like Sorcerer by auto attacking. Brand's overall damage was buffed, so he likely got off his skill more than usual, and just wiped out your champs with his nuke.

Without seeing items, it's hard to say, though your setup was not really great. 2 wild isn't very impressive, sure they both were 3stars, but also tier 1 so not to strong lategame. 3 noble is pretty damn bad lategame 3 bladmeaster is ok, though 2 of them are 2star tier 1s so pretty meh lategame Vi with 2 brawler is fine, but not much more 2star Lucian with no synergies is not strong lategame either And what do you want with 2 lvl 2 garens that don't even trigger the knight bonus? even a lvl 1 morde instead one of the garens would have been stronger to get the kinght bonus

actually, you're quite lucky that draven is broken enough to carry that shit to the second spot, it's a horrible lategame setup...no big synergies(6nobles or 6 blademasters or 4 wild) It's all a bit to much over the place with the 2 wilds, half a noble comp and some hextech thrown in ontop. Not much other syns(no second gunslinger for lucian, gp would have doubled for lucian and your blademasters), a darius instead of one garen would have enable imperial for potetially double damage on draven aswell as the 2 knight bonus Also, no high tier units. 1 tier 4, 1 tier 3, 1 tier 2 and the rest tier 1, the total gold cost of your setup is 57

First place has a 6 brawler bonus, with potentially a carry-bear or mage chogath, and a lvl 3 morgana which is pretty massive. & brawlers alone is already really strong right now, they have alot CC while beeing very tanky with solid damage, potentially even alot if Voli has the right items. Also, you had many not so tanky melees vs a 3star morg, and his Blitz probably pulled in one of your ranged aswell, so morgana alone can basically wipe most of your team. He also had 2 tier 4, 3 tier 3, 2 tier 2 and only 1 tier 1, his total goldcost was 84, he invested ~50% more gold then you despite haveing 1 champ less

Low tier champs are fine early, but they don't transition well into late exept for tiggering high synergie boni, like his WW that isn't strong on it's own but by beeing the 6th brawler he gives 600additional health to all brawlers which is pretty huge.

Lots of disruption for one thing.

Morg, Cho, Vi, Blitz, Reksai and Warwick all do pretty solid CC, on top of being bulky and doing decent damage.

If you didn't protect your Draven (which I'm guessing was most of your damage) you should have lost every time