Mage lootrun build

Heya, I really need a good reliable lootrun build for mage.
I have no idea how the dmg calculation works as it seems that
whatever I create (I have tried all archetypes and different weapon builds) never has
enough dps to reliably defeat the scaled up lootrun enemies.
For some reason I can beat the sky islands expedition up to 100 challenges but with any of the other
lootruns I cant even get more than 20. I feel like its because for example in the SE lootrun the
target destroy missions at challenge 20 already have like 5 million hp.
Im currently not using a mythic weapon but I am willing to invest in one if it creates a set that actually has
enough dps and survivability to defeat the scaled up mobs in all of the lootruns.
My budget currently is at about 6 stx. but Im willing to farm to a maximum of 10 stx to get a set.
Your help is apreciated.