On wowlegacy there is a bug where you can solo Ulduar as a drood easier if you bring some friends as dps. Not sure about other instances/bosses this is the only one I have tried in.
The bosses are not immune to Root/Fear/Stun effects and don't suffer from DR's.
Step 1 Cast entangling roots on boss.
Step 2 Attack the boss
Step 3 When entangling roots buff is almost done Cast entangling roots on boss.
Step 4 Attack the boss
Rinse and repeat till bosses are dead, if the boss casts a spell on you just heal up.
Simple bug is simple.