I found that if I could surround him, then the ones round the back did a lot of damage before he spun round to kill them. Next time I face him I'm going to try and rush guards from the front and some more powerful fighters round the back and see if that works.

-Its helpful to have multiple 3-5 treasuries full of treasure to maximize the boost.
-Gather and or produce many arcane artifacts to boost arcane troops as they have very high combat power.
-At the reward 2 altars (after finishing stage 1 and 2 of the monument) choose rewards that boost the type of unit that you primarily plan to use against the boss.
-Build the Monument of wisdom to spawn ancient defenders who will help you in battle.
-Use the Castle (or even 2) to attack with as many units as possible

It will still take a few attacks but with those tipps in mind its much easier than without :)

I did it, beated him, wow. thanks for your help !

I played this on hard difficulty, there the warrior has 100k hp. It's annoying, as for the whole mission there was no real threat to me, the bandits never really tried to rob the monument, and they didn't send any strong attacks, their numbers were pretty low. I dispatched them with ease.

And then the final boss comes. Even after sending 150+ units (most of them medium or high tier) I couldn't defeat it. With 200 I would probably do it but then between boss activation and the deadline I probably won't be able to produce that many units and will have to go back to previous save, before he's activated.

I just wish the mission was balanced better so that bandits posed a greater threat, making it longer to dispose of them, so that my military and economy was better prepared for dealing with the boss