Actually, if you want an more reliable way to highly increase your attack without reducing your defense like Beserk's Card, you could try the Yellow Crystal 4. It would raise attack MUCH higher than a Red Stone 5 for a boxer and wouldn't lower DF.
It increases ATT by 50% and it's dropped by the Yellow Big Diamond Cactus at Desert 5.
You could try using the Beserk's card, but you'll die very fast and the character will be essentially useless.
Gunner With Sniper Rifle Magician With Ice Meteor Sniper with Pyramid Arrow
And the class I talked about is Boxer
Summarize this with Berserk's card
Also I thought that I will replace my Red Stone 5 with Yellow crystal 4 but I find Red Stone 5 More Reliable because Yellow Crystal 4 gives 50% Atk increase while Red Stone 5 does not only puts enough SP to give a 50% increase but it also gives LP seeing as Red Green and Blue stones add STR DEX and MAG respectively like SP.