Farming Miranda is pain because of how long the dungeon is, even at level 50! Most people that bother to farm here are usually trying to get his jacket, but because the dungeon is so long and drop rate isn't exactly high, many are reluctant to farm him. Please allow Captain Blood to respawn after battles so getting his jacket isn't such a pain.
Typically bosses within instances that have a sigil do not respawn until the subsequent visit as a general game rule in both Pirate101 and Wizard101; however, your suggestion is noted. Thanks!