Ballistas, catapults and tents do not aggro mobs around (may have been changed in a patch), so range fight is the best option (long, but safe).
You better start by destroying tents as they are in back lines, and if you need to cross enemy lines, you prefer them to be at lowaest level when aggroing.
Each time you kill an army member, he will respawn 5 min later with an additionnal experience level. Mobs will always con yellow at lvl 100, but will be toughter, mainly more HP. There are something like 16 levels (title appears in parenthesis under name), the 4 achievments linked to this mission are about killing some of this mobs (level 4, 8, 12 and 16 ?). A 5th ones is linked to killing final golems so they reach the ultimate rank (Supreme god killer).
For the chest looting you will need to bother with the golem pathing, as they will still be active, adding the fact they also respawn after 5 min.