This quest represents the reason you returned to Siwa. It is highly recommended you use sidequests and exploration to level up to or past Level 5 before taking this one on. You can either sneak or fight your way through the Temple- either method is viable, but keep in mind that alerted guards can call in reinforcements, and an alerted Meduman will prowl the grounds with his two lvl 5 guards just footsteps away. Conversely, if you go stealth, be aware that riders on horseback or camelback occasionally come through the Temple, at all hours of day and night. Their speed and unpredictability can catch even the wiliest assassin unawares.
Whichever method you prefer, do take time to carefully scan the area with Senu. Every guard you ID here will remain part of your awareness, allowing you to hunt or evade them as you will.
Medunamun's quarters are in the southeast of the complex. The easiest way to approach unseen is simply to hop up the outer wall to the south.
He sleeps in the house, guarded by his two level 5 bodyguards and a soldier at the door. You can sneak in through the back, but if you wait until dawn, Medunamun and his guards ascend to the roof for some desk time. The stairway up makes a perfect ambush point, as his guards walk in single file going up)
With his guards gone, Medunaman at his desk is easy pickings.
Once he's down, the game will prompt you to confirm your kill with E, but instead this launches you into a hallucinatory dreamstate, then flashback.
The flashback is a year ago, and runs pretty much on rails- listen to the dialogue and follow the objective markers. When it comes to hunting the ibex- shoot from cover. A single shot should be lethal.
Continue on. There will be a brief fight you can't win, and a forced march you can't deviate from, but when it's all done you'll find yourself back with Hepzefa for a bit of wrap-up, and the mission completes.
Note that mission completion ends any alarms raised in the Temple of Amun, but does not resurrect any fallen guards, making this an ideal time to return to the Temple for looting and the completion of any Temple quests that you haven't already dealt with.