I remember when I first started, someone yelled at me for using an LB and not killing them because it makes the boss go into a rage mode. And I believed them for the longest time. Now I use it right after Trick Attack on the second boss. It's been mentioned before, you probably won't have a bar for the first boss. If you wait til the second boss is almost dead, it won't be there for the final boss. So I go ham. I have yet to see a castor use it on trash mobs though. I should get on BLM to do it and get yelled at now.