Best chance is probably to save all your gems and buy the 900 gem pack with 11 ancient shards. I would guess there is a limit to how many ancient shards can drop in dungeons a month. Ancient shards also drop from Clan boss, even at really low levels.I'd like to add that it also dropped from the Clan activity chest for me a week or so.
Outside of '2x Shard pull events' your chances of getting a legendary from an ancient shard is 1 in 200, and from a sacred shard the chances are 1 in 16.7. After that they have a mercy system where your chances increase for every pull until you finally get one, then it resets back to stock chances.
It all comes down to luck unfortunately...