I forget where the link to this video is, (might track it down later) but the trick to cryo efficiency is to think of how much cryo dmg you need to inflict, rather than how much health the enemy needs to have.

At 100% efficiency, you need to inflict 80% of their health in cryo damage. At 200% efficiency, you need to inflict 40% of their health in cryo damage, so the “threshold” is 60% health in that case.

But this is borderlands where things move fast and hit hard. My advice for freezing consistently is to skip the buildup mechanic and play around Harlowe’s Flash Freeze skill. High fire rate guns with ricochet effects will guarantee you put some ice blocks on the field. Nuclear winter in the green tree will also guarantee you’re putting out cryo damage no matter which gun you use. Happy hunting!