It should be pretty easy, go to your clan page and mouse over the exclamation near the stars at the top right.  It will tell you what you need to do.  I dont have the game up now but I know one is to use 2 clan boss keys and another is to fight in 12 classic arenas.  I forget the others.

Just checked, it is on the members page of clan tab the 4 quests are below.  Make sure you click the check in button on the same page too for credit to count.

Play the game for 90 minutes (You can just leave your phone with it on idle doesnt need to be active play)

Battle the clan boss 2 times

Fight in classic arena 12 times

Fight in a dungeon or campaign battle 20 times.  

Play the game for 90 minutes     done -  1 stars

Battle the clan boss 2 times    done  - got 1 star

Fight in classic arena 12 times   done - 1 stars

Fight in a dungeon or campaign battle 20 times.    done 0 stars

thanks

You can finish this in one day, easily at level 5 if you are in a clan. Dosnt even have to be wins... just fights.