Well, there are some key epics for Mage you need if you're committing to that archetype. You definitely need an Ice Block and two Primordial Glyphs more than you need a DK. Maybe post what your deck looks like now so we have an idea of what you're working with/toward?
### Real Magic
# Class: Mage
# Format: Standard
# Year of the Mammoth
#
# 2x (1) Babbling Book
# 2x (1) Mana Wyrm
# 2x (2) Arcanologist
# 2x (2) Doomsayer
# 2x (2) Frostbolt
# 2x (2) Medivh's Valet
# 1x (2) Shatter
# 2x (3) Arcane Intellect
# 2x (3) Frost Nova
# 1x (3) Ice Barrier
# 2x (3) Ice Block
# 2x (4) Fireball
# 2x (4) Polymorph
# 2x (6) Blizzard
# 2x (7) Firelands Portal
# 1x (7) Flamestrike
# 1x (8) Medivh, the Guardian
fwiw I did have and play through Normal Karazhan before (hence Medivh and Valet), but I was only doing PvE stuff in the game and have only recently gotten into PvP