1. Get a character saved up to the boss you want to do on multiple alts (if you've got a lowbie alt you want the save on - you can share it from a level 85 - get a friend to "boost" the save character). Obviously it's easier to work on 12B or Handmaiden/Blazing, otherwise you have to do Blackhorn/Spine on each character as well.
2. Log the save character (and extend the raid lockout if you need to) and make a premade group (I suggest under legacy raid, select Ruby Sanctum - less likely to get a realm hopper). Make sure you set the right raid difficulty, and set it to auto-accept.
3. Log another character, join the group, and do the boss you want!
4. When you've done it on one character, don't leave group! Log the next character you want to do it on, and join the group again. While travelling to the raid instance, the current character will be given leader - before entering the instance, kick the last character (but not the original character from step 1) and then right click your portrait and Reset Instances.
Repeat step 4 for every other character
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