For this trophy you will need to find a blood gem with a rank of 15 or higher. The best (and possibly only) place to find said gem lies within chalice dungeons with a depth of 4 or 5. One of the earliest spots to get this trophy is in the initial Ailing Loran dungeon. Once you beat the first boss, he has a chance to drop a blood gem of rank 14, 15, or 16, from what I've seen.
However, if you intend to platinum Bloodborne, you will earn this trophy automatically while looking for Red Jelly, which is a crafting material needed to open the Pthumerian Queen's dungeon. The best location to find a rank 15 gem and Red Jelly is the Great Isz chalice dungeon. I looted half a dozen high ranking gems from normal enemies and the bosses in one run alone.