EcchiOtakuTM

Pure frost is very fun to play in RPvE 9, and I assume in CPvE as well, but for that we need to call @Dallarian
Anyway, this is the deck I use for RPvE 9:

I think Dreadnought is awesome for CPvE but I never played pure Frost before just because of the prohibitive cost. I am just (re)starting Skylords myself and I am still going with my old favorite of Stonekin but doing Nature start instead of Frost start this time around. I like Frost myself but Stonekin was the cheaper option to start with and allows for working towards pure Frost (or Nature).
 
With that said, I would throw other expensive options out there of Enlightenment and Amii Monument (I never owned the latter).  Still trying to decide that my "endgame" would look like (definitely finishing all maps on expert would be on top of my list; I think I was 4 to 6 maps away). I guess I'll see what daily Stonekin boosters toss me and go from there.
 
 

Pure Frost has become a quite decent deck since the birthday patch, but it is one of the more (if not most) expensive decks around. 
An endgame viable Frost Deck contains of more ultra rare cards then any other deck.
Though it is expensive, it is hardly suiteble for BG 10s due to lacking mid tier tools, mainly in T2.
I personally would recommend to play splashed decks from beginner to advanced player levels. Pure decks can be fun and decent, but are usually more expensive and need higher skill levels (for harder tasks).
How good Frost works you can read here, a great guide from one of our most dedicated Frost players:
 
This is a guide how the expert maps can be approached, sometimes in Frost perspective too if I remember correct:
 

A completely different deck than Volin showed. 
This deck is somewhat popular in RPvE 9. The goal is to summon a couple constructs and then warp them from camp to camp, annihilating everything in the process. 

You do not need the promo for this deck, but since you asked "endgame" I had to include it. 

I can throw in my Lost Souls rpve 9 allrounder - not fully optimized but strong and fun

 
Edit: Blue start for the fun btw, I had not much blue starts when making this deck. Same goes for the Dancer T2, this is not a meta approach, but slotted for fun and diversity. A double shadow start would be obviously better as usual in pve

The faction ability from Lost Souls (Frost/Shadow) is sadly not very good, the combination of elements is still pretty good though! For endgame content, it really depends what you want to do. For PvP, you already get the free PvP decks, so you would only need the few cards you might want to swap. For PvE, most endgame content is rPvE, where the focus is on killing things fast, where the campaign sometimes has more variety in goals. 
Infect is considered one of the stronger shadow cards, but its also one of the most expensive. Cultist Master (in combination with Furnace of Flesh) is also usefull in multiple situations. Overlord and Ashbone Pyro are also good cards to have in your collection. 
Maybe @Dallarian has some suggestions for Frost 🙂