What current Playlist can I find Stronghold game mode that counts for Castle Crasher achievement?
Team Arena. The playlist also includes Slayer, Oddball and CTF game modes, so finding Strongholds may take some time.
For the next 2 days you can get it in hardcore fiesta. You can also get it in Super fiesta party but it would be extremely rare.
The main playlist would be team arena as already mentioned.
When core play comes into rotation you can find it there too.
Lastly, big team battle, but rare.and good luck getting a game in the first place.
Team Arena, and the limited hardcore fiesta.