The climb from the east to Frosty Mountain is only for the hardy. It's nearly 7.5 kilometers of uphill, with the last kilometer in particular getting really tough once again. The advantage in the cold season: you climb the sunny side, which will be much less snowy. You will also enjoy amazing views of the Cascade mountains in the US.
The ascent from the west, on the other hand, is not as steep. Also, you can spend a night at Frosty Creek Camp at about 1,900 meters and climb the summit the next day. To spend the night in the backcountry, you'll need a permit that costs $5 a night per person.
Also note: In winter and even in fall, Frosty Mountain can be snow covered. So if you are descending to the west, you should have microspikes in your backpack for this case.