Bio does not work all the time, it should take about 4 tries at most to poison Seymour, but it does not do 25% to Seymour as it does to your characters. It does about 5% (which is still more than anything you can do at this point).

The Mortivessel regenerates in this pattern 4k (starting), 4k, 3k, 2k, 1k. So either use the Aeons as early as you can (causing 9999+4000 to Seymour), to avoid the declining returns, or use them only for the Overkill.