Loyal Halo fan.
H:CE, H:CE PC, H:2, H:2 Vista, H:3, H:3 ODST, H:Wars, H:Reach
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For locations watch this. If your problem is actually getting to them then watch carefully on this video or others on how he gets to them.
Cortana's skull isn't too hard. Make sure you walk back as far as you can on the jumps up overhangs and take your time. Also if you are not crouch jumping try that.