Have you seen the TV Show TOm & Jerry, silly me of course you had. See, GKz are like Jerry they attack, then defend, then attack again when they can, then defend. Means you must be able to guard yourself when you have lack of drawing monsters with enough power to break through your opponent's. Even tho Descendant is a key card to accomplish that, without Spy, It's hard to get it done in one turn. Especially if your opponent already has monsters with high attack points. Therefor there must be some sort of a balance in your deck, Monsters should be able to attack when needed, and defend as well when required to. Spell cards are there to make your life easier, enables you to draw faster, such as duality, cripples your opponent, such as necrovally and royal tribute.
Other cards like Night Beam, Assailant, Swords of Revealing Light, Magic Dimension, Rite of Spirit, Spellbook of Wisdom, and United we stand are all cards you won't be needing. All you need to focus on is get that necrovally, set that spy, and few backrows = gg's. Of course with a little of intelligence of where to use what.