Warning: This cheat can freeze your device on occasion. But it definitely works... until it's patched, that is.
In the energy menu, selected by clicking on the lightning icon below the boxer's portrait in the upper lefthand corner, there will be a "sponsored" free energy bar that gives your chosen boxer +10 energy. Though you will have to watch one, maybe two commercials, in a row, tap on the button that says "Free" but continue to tap that area furiously even while the screen fades to the black "This Item Is Offered By..." screen to upload the ads. Once the ad starts, though, stop clicking on the screen immediately.
If done properly (I would say it's about a 75% success rate), once you finish watching the ads, you will earn +10 energy for every time you click on the "Free" button. This makes it easy to enter fights in quick succession and to train your boxer without having to wait hours for the boxer's energy to recharge naturally.
General Tips
1) Yes, the game is unfair.
Let's be honest here: The game is incredibly unbalanced, as it will force you to fight boxers with ridiculous items (and the only counter is for you to use premium currency to match them). If the opponent you face has a 100% KO Restore or a stat-boosting item, you're pretty much screwed.
That said, don't be afraid of losing here. If you can force the opponent to use one of his items during the match, there's a chance that the next time you face him, he won't have that item or ANY items in the rematch. And hey, even if you lose, you still earn ranking points on the tournament ladder. So it's not a total loss.
2) Focus on Attack and Stamina, maybe Strength.
Don't bother so much with Agility or Defense. Over the long haul, just speaking from experience, these two stats just don't contribute enough to matter. Attack is far important in dealing damage and Stamina increases your HP and your KO restore when you select defensive strategies. Strength can be a good secondary stat if you want an extra edge, but there's a reason why the higher-ranked players have boxers with a ridiculously high attack and stamina rating with almost no attention given to the other stats.
3) Bet on Attack, Stamina, and Strength.
In fact, when you have the chance to bet on fights, always go with the boxer with the higher overall Attack, Stamina, and Strength. Don't look at the odds so much as they can be off. More chances that most (for me, I get it right about 90-95% of the time), the one with the higher stats in those three categories will win the fight. For pairings where one boxer is clearly ahead of his opponent, just bet the max 20,000 credits. If the boxers are even in those three stats, just bet 1,000 to be safe (and just to clear the board so that a new five bets can be placed the following day).
4) Go on the offensive, or switch between defensive and balanced.
I tend to go with two strategies in a fight. The first is just to go right in for the kill by clicking on offensive and knocking the opponent down as much as possible before the offensive option is grayed out. From there, I switch to the second strategy which is to switch between defensive and balanced tactics, with the "balanced" tactic hitting the opponent and then the "defensive" tactic defending against an opponent's blow.
To be more specific here, the "balanced" tactic automatically puts the battle into a mode where you and your opponent both trade hits one for one. So if you time it right, you can soften the blow of an opponent by switching to "defensive" (you always get some HP back for choosing "defensive) when he hits you and then switching back to "balanced" to hit him the next turn.
