This is an easy mission maybe the easiest of the whole game. Only Trolls will attack you and they are quite weak and even in close combat easy to defeat. The long range weapons should be placed in a way that they have a free line to the trolls. One direct hit will kill them. Mages are not effective against them but sometimes the spells work and the troll will get huge damage.
The Staff of Osiris can be found in this mission under the stone in the north west corner of the map (an arrow made of stones shows you the position).