Veywild Shield, if you have it, has Freeze effect potency. Frostwyrm has generic status potency, so it'd work as well.
Frost Effigy is just a better version of Nilak with the ability to inflict Freeze on its own.
I don't believe there's any weapon that's better for Freeze. Spacefarer's Cannon has a higher chance in its Freeze mode, but it's a 100-proc Ranged weapon and rare.