I’ve done some testing here, with 5 points in Bitter Winter for 100 freeze rate, my necro has 998% freeze multi, ~40 flame wraiths (attacking at roughly 1 attack per sec) & 4 fire skellie mages. With ~40 hits per second I would expect to freeze most things, but the last monolith I was in had a primeval dragon as the boss (lvl 62) and I only froze it once when it was less than 50%.
Edit: I have +29% inc health for normal mobs & +130% inc health for rare mobs if that helps.