The basic rotations are:

A: Howling Blast > Frost Strike > Necrotic Strike > Blood Strike

This is AoE (the Howling Blast) and damages several opponents at once.
Frost Strike is a priority over Unholy/Blood runes, because of its chance to regenerate a rune.


B: Obliterate > Blood Strike > Frost Strike

This is entirely single target damage, but benefits from Outbreak applying diseases to the target first.
When hitting a single target, this hits much harder than the Howling Blast rotation.
And the Hungering Cold on round-robin, to CC a lot of the other team lets you pick and choose who dies while the others are helpless... (HC > They Trinket > HC.... and two more HC's to repeat in 30 seconds, after Diminishing Returns).