Respawn Raid Boss

Once you've killed him the first time, just have everyone drop group (it will automatically port you out of instance). Once outside, reform the raid with the same groups and raid leader, and zone in to reset instance.

If anyone can confirm this works on other boss's too please post it in discussions. Kylliki seems like it resets also but causes issues at times (split groups), while Vistrix seems to keep its lock.