Maybe using an elemental instrument to provok bloods onto it, even then the chance is really low. Tamer is probably the only way.
Slayer instruments to provoke onto other blood elementals. Idealy if your bard has discord, get one to land first to make provo easy. If no discord, provo dexxer with a cheap blood ele slayer will do some serious damage so you dont have to provo like 10 bloods to kill it, more like 1-3. Poison field directly on a casting mobs forces it to cure itself before casting dmging spells, Im not 100% sure poison field works on diseased bloods, but it does work on most high end mobs where a direct cast posion wont apply. If poison field works to keep it curing, it makes it easier to swarm a casting (dispelling) moster with blade spirits to take it down as well.
Yeah personally I lure the Boss up to the Posion els area...Is 2 poisons and a acid el...….The boss Auto attackes the Poison and acid els(no need to prov) If you can land a discord on the Boss its much easier..If not you may have to heal the Poison elemental