first of all as an info: this dungeon resets only once a day with the daily Server reset, not like tt2 every 30 minutes.
there is no Point at all doing it at lvl 41 as you Need 4 quests, and if my Memory serves me correctly, then some of em you get only at lvl 42.
if you die at olivier zillions of time, the good News is that the next Boss as a mage is easily to do cos you just Pop reflections when he does that spitting on you

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now the really bad news: after this Boss come some demons, surrounded with 2 to 3 cultist. with lvl 41 mercs your Rowdy will die after about 5 seconds, there is nothing to do about it. with a Scout lvl 8 runes and egg buff it was almost impossible to kill them, and there is 5 or 6 of them before the final Boss.
there is one Thing i do in General. you get those free premium crystals while dinging every 5th lvl or when finishing very important wm quests. do yourself a favour and buy this grain of blessed francincense so that you dont lose xp from dieing. i always buy 1 and have it active.
here some things that you can do at Olivier:
he puts some purple circles on the ground, move out fast they do good dmg. move your mercs out with this Splitting skill.
always be near the couldron and hit this skill in order that you can kill those shadow fighters. at around 20 or 25% of his health olivier will become shadow himself, so you and your mercs can only do dmg on him when you use this couldron buff.