Hidden Objective(s): Sink the Heiei within five minutes; When you get reinforcements the Heiei will show up right behind them soon after. Both of the ships have torpedoes to swith to both of them and use the torpedoes on the Heiei (Nothing else really works because it is much more powerful then both of the ships). A few good hits should sink it. If you do this within five minutes you will have completed the hidden objective. NOTE: Even if you fail this objective it still becomes a primary objective, so you're going to have to sink it one way or another.
Secondary Objective(s): Don't lose five or more ships; using your units cautiously and keeping them together can help you achieve this objective.
Complete both of these objectives and the primary objectives will unlock the Iowa class battleship