Not sure what boss you’re at but there is one in 2 that is extremely challenging and I usually come back to later on - much later. It seems logical to do the archstones in the order going up the stairway in the nexus, and that’s not entirely wrong, but you don’t have to complete one before moving on to the next (and in fact can’t as @Daggers has pointed out).
If you are struggling on one, you may find you can make good progress in other levels instead and you may find better weapons / loot in them too. .