I have tried for two days endlessly strategies to do the bonus objectives on extreme, if I focus on the army I got low on food, If I focus on food I don't have enough time to do the objectives before the mercenaries arrive, If I try to do things fast I get low on food again, If I try to only destroy the outposts I can't get to Hegen, somebody has any solution?

If you're trying to get the achievements or the skins for the Warchiefs, you can finish the bonus objectives on normal.

I use a good guide for the Extreme mode, though it doesn't focus on the bonus objectives, though it is very helpful.

https://steamcommunity.com/sharedfiles/filedetails/?id=1334570143