Quite a few points can improve freeze buildup:
- Change weapon rune from chaos to cold
- Get a weapon with % increased elemental damage instead of flat phys
- Get some flat cold on your rings
- Respec ascendency to "I am the blizzard" instead of "I am the thunder" (debatable whether that's better in long run)
- Get "One with the river" passive node while going left side - gives good buildup + defensive stats
- Time to upgrade your gloves as well - phys leech to mana is taken care of by the ring, and to health is useless - try to get +2 levels and at least some flat cold damage in there.
- For whirling assault change heft and heavy swing to elemental armament 2 + magnified area 2
Just a couple of these points should be enough to make cold damage your main damage type and get your to proc herald of ice easier