Even thought I can stop all melee attacker, but Stormcloak archer...and our own archer can destroy the barricades easily

Well, actually the barricades was always destroy by Imperial archer.

Is that possible to complete first task?

I managed by running towards were they spawn (near the farms) and spamming health potions. They try to kill me and never even go near whiterun's gates. Depends on the build i would of thought, as im a battle mage so had ice cloak/ice storms and heavy armour.