Across the Obelisk

To get "Stack X Condition" achievements any single monster or hero needs to have the required condition cumulatively stacked up on them.

I found that stacking 200 Chill charges on a monster was very difficult and I was likely to accidentally kill the monster prior to reaching 200. Instead I opted for having 200 Chill charges applied to one of my heros. This can be done in Act 2 or 3, I did it in Act 2.

The concept is simple, head to Faeborg (the boss of the blue map), keep one of you heros alive for long enough for Faeborg to stack up 200 Chill charges (it took me about 10 rounds). Faeborg's moveset is based upon applying Chill to all heros, applying additional Chill to the two middle heros, and then applying damage based on how much Chill is accumulated; meaning your heros become far more vulnerable (and slow) as the fight goes on. Faeborg does a lot of damage when you have over 100 Chill stacked, so you need to build up a lot of Block on the whole team. Since your team gets slowed down in the innitiative you will need to have Fortify covering the whole team. As one of Faeborg's abilities stacks Chill on a random middle hero, it's good to limit the options by having the other middle hero invisible. This is the team (from front line to back) I used and why:

1: Heiner: Supply massive amounts (hundreds) of Block on all heros, and maintain Fortify so the block does not disappear. Get some cards that apply Fortify to all heros.
2: Magnus: Target of the Chill stack and the one you need to keep alive. Also stack block on self and other heros. An Entrentch card upgraded for fortify goes a long way. Magnus will likely spend quite a few rounds petrified, so you're likely to build up enough energy for a card like Citadel if you come across it (I didn't need it).
3: Thuls: Keep him invisible and skip all of his turns. This will focus all the 'middle hero' Chill effects on Magnus.
4: Reginald: Reg is here for Heals and dispells on the team. Be sure not to dispell the chill on Magnus as that's what we want, so Mass Dispell cards are not useful here.

Note: Do not build Heiner or Magnus for Thorns, the first time I tried this achievement I accidentally killed Faeborg just with thorn damage (this includes the Shield of Thorns you get from Ylmer)