This shirt is rewarded for beating Razorgrin, a Rank 5 brawler's guild boss.
To be able to fight him, you must fish up Impeccably Sharp Tooth which begins the quest Razorgrin. This allows you to purchase Challenge Card: Razorgrin from the NPC and turn it into to a guard to challenge the shark.
Ranged should have no problem defeating Razorgrin - just make sure you keep away from him. His fierce bite will one shot anyone who comes too close.
For awhile, this fight was nearly impossible to beat as melee because he would instagib you when you got into melee range and scant ranged attacks weren't enough to beat the berserk. However, he has been fixed so as long as you DPS him from the tail, you can avoid being bitten. He will flop around, so make sure you are paying attention to which way he turns so you can turn too.
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