Swiftshot. As permafrost requires frost to be applied, it is weaker than your nothern winds. I do agree on the frozen mallet, however. Not a useful item for an AP off/full tank

Sejuani has no reason for a frozen mallet her passive mixed with perma frost is plenty enough to keep someone pinned down long enough to kill them. I Would also recomend permafrost as your first abilitie insted of northern winds it does a little bit more damage to everything.