This is what I am currently using. It seems to be doing ok for me.
#showtooltip Howling blast
/targetenemy [noharm][noexists]
/castsequence reset=10 Howling Blast, Plague Strike, Soul Reaper, Frost Strike, Howling Blast, Howling Blast, Frost Strike
/cast [combat] !Plague Leech
/cast [combat] Pillar of Frost
/cast [combat] Outbreak
/cast [combat] Empower Rune Weapon
/script UIErrorsFrame:Clear()