Level 7 brings about good additions to your arsenal, while the mobs are getting harder and more of them are now hostile to you on sight. Stone Skin provides some damage absorption and Frozen Shock is your second chain spell. Triggered by Ice Chain, Frozen Shock adds a brief knock-back and stun to your opener, giving you even more room to charge up those Flame Bolts.

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You may be tempted to use Stone Skin as a precaution, applying this spell before
battle begins. However, you might consider that if you draw the attention of a second (or third) mob while it's still on cool down, you may be in trouble. Rather, keep Stone Skin hot-keyed in a handy position so that you can pop it when you are actually taking damage. It will probably hold long enough for you to crowd-control an add or even flee, if necessary.
Your rotation is now Ice Chain, followed by Frozen Shock (when available), Flame Bolt, Blaze (when available), Flame Bolt. Hang on to Erosion for the time being, as it really doesn't have a handy place in your spell rotation unless your chain spells don't activate. Otherwise, using it early in the rotation will lessen the stun effect of your frost spells, and saving it until later won't give it the time it needs to be effective.
Level 9 only gives you one spell, Energy Boost, which returns a little MP to you. Use this ability whenever the cool down is up in order to keep you casting and moving with a minimum of down time.