End game build for [Frost Prime].
Disclaimer: This build requires 5 normal Froma and 1 Umbra Forma.
It's just not worth it to use an Umbra Forma on him at the moment, so do it only if you really enjoy playing him (I do, but most people don't).
I like using [Rejuvenation] with [Frost] but the aura mod is totally up to you.
If you don't have [Primed Sure Footed] you can use a V polarity on the exilus slot and use [Cunning Drift], because it adds a bit of extra range and mobility to [Frost], who is a slow frame. I really recommend using PSF, tho, because CC sucks and [Frost] has no way to suvive if he gets chain CC'd.

Here's my [Frost] build. This build focuses on the defense of objectives, with enough efficiency to be able to recast the Snow Globe (3) enough to follow a moving defense objective, such as the Arbitrations guys, and enough strength to be able to use properly the armor reduction from Pillage (2) and Avalanche (4).

Let's take a look at his abilities:
Passive: enemies that hit you will be frozen. Recomendation: Don't use this. If they are hitting you that means that you will die, even with 90% reduction from [Adaptation] and almost 900 armor.

His Freeze (1) should only be used to pop the Snow Globe (3), because it deals no damage to any relevant enemy past level 10.

Pillage (2) provides [Frost] for some extra survivability (replacing the really low damage and irrelevant slow from his original 2) and [Adaptation] when you have to get out of the bubble or when moving from one location to another.
Pillage also removes Status Effects, making it really good to cleanse toxin or magnetic procs, and reduces shields and armor from enemies.

Snow Globe (3) is a frozen bubble that stops incoming projectiles from any source, including most allies' (13k hp without other modifiers), which shouldn't be popped unless it's nullified, and is big enough to comfortably move and shoot inside. It can also be used offensively to push enemies outside of the area, and if they hit any obstacle they can receive up to 50% of their max hp as true damage. 50% of the enemies that touch it will also get frozen due to the [Chilling Globe] augment, and enemies inside of it will receive a 70% movement speed reduction.
The bubble is invulnerable for the first 4 seconds after it is cast, and if it's casted while inside another bubble it will absorb the remaining health and add it to the current snow globe health, almost infinitely.
This means that if they pop it you are either not using it enough or doing something wrong.

His 4 also gives a 70% armor reduction, and can be recasted due to the low energy cost. It doesnt deal much damage (oneshots until round 10 or so on hydron, but that's it, some of the stronger enemies will survive 2 or even 3 casts), but it really helps the rest of the team to kill all the enemies in the area, and temporarily freezes them.

To sum up:

1 is for popping 3 ONLY.
2 is for survivability. When cast with 4 will reduce the enemy armor to 0.
3 is for defense. The HP of the bubble stacks almost infinitely and is invulnerable the first 4 seconds, so spam it.
4 is for reducing armor, and freezing enemies.
If you are using [Frost]'s passive, it's because you aren't spamming the 3 enough.