It's a free (almost-)tink bag: it's supposed to be hard
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But as the person who killed over 500 of those sharks to arrive at that wiki number, I can assure you that the game's random number generator is streaky: you can
absolutely go for 20 (if not 30 or 40) non-nameds in a row on a spawn point that has a (roughly) 1 in 10 chance of producing a named.
Just keep killing and eventually you'll get your chest. Loramin is carrying around like five of them, and I've given a couple to alts and sold some also; you just have to commit to the camp for the long haul (long enough to make a 500-entry spreadsheet ...)