Water/Spirit or Earth/Spirit is a pretty big hole in the armory. I filled that as soon as I could, but with a Legendary+ boss armour.

And they are not avoiding any elements by building up the big four. One of them can power hit any dual/element boss without taking bonus damage in return, except earth/water.

You will be able to crush the swamp kraken long before you can beat the epic flavour of the week to 43. Getting a legendary + armour is what most of the people care about in regards to the boss: The 4 fusion stones, 57.5K gold, 11 keys, 4 gems and handful of low ep armours are not interesting. The 5 DPC keys used to be a nice bonus, but I think that they are too expensive for any one to 'farm' any more.