I've formatted my external HD with BTRFS (aware of the risks). What are options I should use to increase failure tolerance on sudden power loss to the drive? 
Mount options for BTRFS on an external USB drive
btrfs
main danger if you would use mounting option notreelog which must be avoided in case of external drive. Also using of old Kernel versions increase the risk.Other significant option for performance and also data safety is:commit=number (since 3.12)  Set the interval of periodic commit, 30 seconds by default.So to sacrifice performance but increase safety usecommit=1 in mounting options.Also I usually add noatime if there is no need for such information, which removes unnecessary write operations on file access (read), thus reducing risk.One more good option is recovery, which enables automatic recovery in case drive WAS unplugged or just lost the power.