Wouldn't it be even faster and just as cheap if you just shot dragon twice with your bow? Dragon is dead, no need to play with potions or poisons.
My preferred and most optimal build imo is currently: either elven armor with nord hero weapons or glass armor with Chillrend and shield (+1 perk, you gain highest 1h damage and maxed armor with only 1 point in LA)
Your got the wrong idea, you think the mage is using exploited gear and pots and berserker rage and even then it shouldnt be posibble to one shot an ancient dragon without sneak bonuses,
The mage im talking about dosnt exploit anything except alchemy to make himself pots and poison, still without even that he could vendor buy a 50% dmg destro pot and 100% weakness to magic poison.
The bow can be the lowest non-upgraded stock weapon with the lowest paralize enchant on it and using only steel arrows. It is not the way of dealing damage, its a way of poisoning the target without the need of being discovered or getting in range to poison it with a sword or picpocketing with a posion.