Does this achievement (as well as all the other kill X heroes/nemesis/enemies) go by a single character’s stats or is it accumulative through all your characters?
It appears to be bugged as I have Mass Extermination, but not Ocean of Blood…
Edit: Looking at the global achievement statistics, it appears this is bugged since only .2% of players have this achievement whereas .8% of players have the Mass Extermination achievements.
For reference, Mass Extermination = killing 750,000 enemies whereas Ocean of Blood = killing 500,000 enemies.