Myth of the Dawn

For this achievement you only need to have the stated itemlevel on average in your bags, not even equipped.

The macros are only useful for the lower achievements. Don't get confused by the macros being posted here.

For discount Upgrade purposes, this achievement works per slot after you get it account-wide.
So if one of your chars have a 289 helm equipped, all of your other toons only have to pay 10 Mythic crests per ugprade for helm slot.
If your toon still has say heroic track rings at 276, your other toons would still have to pay 20 Mythic crests per upgrade for rings
Once any toon on the account upgrades their rings(Both I think) to 289 all the other toons would get the discount for 10 Mythic crests per ring upgrade.

Progress check macro that prints sufficiently upgraded slots in green:

/run local n=tInvert(Enum.ItemRedundancySlot),a;for i=0,16 do a=C_ItemUpgrade.GetHighWatermarkForSlot(i);local c=(a>=285)and"\124cff00ff00"or"";print(format("%s%d\124r %s",c,a,n))end