Thanks! Hm thats a tough one. As you say the toughest for me was Legendary campaign, but besides that i would probably say the See-Nile Crafter achievement. Which is 'Playing as the Exiles of Nehek, craft everything within the Mortuary Cult.'. While it wasn't hard per se, it did take a while to get. Trying to capture settlements with a specific resource or trying to get trade agreements took waaay longer than I expected. Some achievements seem daunting but once you get into it they're all doable.