I have a level 50 with balance and death. I was surprised by how powerful I thought it was, until I recently hit 48 on my Death main. I did have him trained in lightning w/ all balance tree traps and blades.
Lightning gave me strong spells, but really left me in the dark as a secondary school with only frost/fire shield. So I switched from Death w/ lightning to Death w/ life, with only the life/death/myth blades and shields, (from the Balance tree in Krok). Did I make a mistake?
My question is what is the best secondary school to accompany Death as your main school?
Take Ice up to Tower Shield. This will also give you the Fire/Storm shield.
Pick up the Spirit Blades and the Spirit Traps from Niles, the Balance tree.
Take Life up to Sprite (or even go for Satyr).
For defense, this gives you Life/Myth, Death/Myth, Death, Fire/Storm, and Tower Shield. Always keep the max amount of Tower Shields in your deck, then add 2 or 3 of one of the other shields and switch it out as the situation demands.
For healing, you will now have you Pixies, Sacrifice, Sprite and possibly Satyr. We all know that you can heal yourself when you attack with Ghoul, Vampire, Wraith and Scarecrow, it never hurts to have the ability to heal others as well.
As far as attack cards, I never have anything in my deck other than my Death cards. The damage bonus from your equipment, in conjunction with being able to take advantage of your powerpips makes using attack cards from another school detrimental.
After all, you choose the other schools for the cards that fill out the areas you were lacking, such as defense and extra healing for others.
Set up a 2nd deck specifically for fighting Death monsters. Max out your Death Prism, have blades, traps and feint, Wraiths and Skeleton Pirate. I also buy 10-20 Death Prism treasure cards from the bazaar and fill my sideboard with them. The nice thing about fighting Death monsters is that they rarely shield against Life, so your Death Prisms are almost always effective.