Educated Guesser

I've found that the best way to get this achievement without having to cheese it is by doing exactly what the achievement says - make smart, educated guesses.
Before you start betting, spend a bit of time getting to know the brawlers. Who are the rank 8s doing random brawls? Who are the raiders blasting through the ranks? Will that person have enough DPS to make it past Nibbleh? Has that person been reading up on the fights ahead of time, or are they going in blind? Knowing things like this will help with the accuracy of your bets. Here are a few things I noticed when getting my achievement:

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• Betting on the hero fighting Oso is almost a surefire stack, plus you can use that fight to gauge how good of a player they are. Watch their DPS and how they move. If they seem like a good, aware player, they'll most likely blast through Rank 1.
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  • The biggest roadblocks for new brawlers seem to be Grandpa Grummlefloot (Rank 1), Master Paku (Rank 2), Burnstachio (Rank 4), Thwak U (Rank 5), and everything in Rank 7. If it's someones first time doing these fights, they will most likely die.
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    • Abstain from betting if it's someones second time (or more) doing the fight. There's too much uncertainty with the outcome, as that person might be a quick learner, or they might need a couple pulls to kill that boss. Everytime they fight a boss, their chance of success goes up.
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      • Always wait to see who the opponent is, especially for random brawls. Someone going up against Unguloxx will win almost all the time, but if they get Hexos... well, it'll be a quick and easy stack for you.
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        • Do not be queued while you bet! By not being queued, you won't get pulled in for rumbles, which are practically impossible for the heroes to fail! Plus, I don't know if this is a bug or not, but correctly guessing the outcome of a rumble will give you two stacks.
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          • Check people's gear before you bet on them. While gear is obviously not correlated with skill, I feel much safer betting on someone with a 904 artifact and two BiS legendaries over someone with an 824 artifact and no legendaries.
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            • Know the fights, and know your brawlers. Sure you could probably get this achievement betting blindly, but it will go much faster if you actually know how the fight will play out!
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            Most importantly, remember that you do not have to place a bet every single fight! If you feel unsure about a fighter, it's better to not bet at all, rather than bet poorly and lose your stacks!
            Happy betting, and good luck!

Before you can place your "wager" with [npc=68365] (he will gain the [spell=230029] buff once the item has been turned in), you have to purchase a [item=143762] from [npc=118898] for 100 Brawler's Gold and bring it to [npc=118904].

edit: this is for Alliance of course, Horde would be very similar, just different NPCs

Quick tip : If you leave the zone, you lose the buff!