That was the only challenge i could not do as well.  I also paid for the BP.

I eventually got the 5th rare but it was painful, cost me all the gems in refreshing the market (was saving them for rare shards), and dedicated all my energy on attacking stage 7 of dragon so I could try and get mystery shards. 

I havent used the void shard yet, too worried that is will be rubbish and all those hours spent were wasted. 

Sorry about your luck.  I bought the battle pass 7 days ago (when i started the game) - so technically when week 8 opened up.

I got 5 rares within those seven days.  I do play a lot and open many mysteries every day to level through campaign as food, but i expect an end game player to have much higher access to these than me.