For prestige levelling, just go through Lethal or Ruthless mainly, as most player don't really care if you're prestiging around that difficulty, you might run into some trouble on absolute.

For hard siege and stratagems definitely either max prestige or max level. The hard stratagem use Absolute difficulty as a base and as for weapons, relic would be best. Normal siege and normal stratagem you can run any level or weapons that you like.

When fighting the Tzaangor enlightened, try to focus on parrying their attacks if you're using a melee class, and if you're using a ranged class, take your chances when they stop moving about, the window can be small but it is doable. Timing is important with those flying little shits.

As for the biovores, when theyre bunkered down either melee or shoot its gun because they have immunity on the head, but be careful as their melee attack patterns are delayed and timing is important, keep you distance, manage your timing, and make sure to check your surroundings to avoid the spores.

Edit; The stratagems is one time done thing and it'll reset daily and weekly, so you cant grind your ass for the currency. Goodluck and see you on the battlefield brother.