The thing is most people dont relize with the right gear and right masteries u can build a team around any champ the thing about raid is and what zI suggest to everyone is focus on one champ at a time before you move on to another champ or before you pull more shards because you start working on a champ but you win a few  shards from quest or events and then you open and you see someone better you are going to start working on them  and then when you get more shards you open and switch and before you know it you have alot of champs that are halfway done. I suggest pick one champ, level and rank that champ to 60 then grind potions to acend and then fully master before you move on to next champ. I did not include skill tomes and gear because skill tomes are hard to come by and gear you are going to be constantly switching out and changing. but save every shard you get until the champ you picked is a fully acended fully mastered lvl 60 6 star champ then pull shards and pick the next champ to work on. I swear in the long run if you plan on sticking with this game you will be so glad you did.  

Honestly I have about 13 Legend and even though I have the champs to make a ultimate Clan boss team , doing  more than 5 mil damage with one key is a struggle  for myself

My point being, there are you tube videos of beating much of the game with Rare  champions with the right gear. With the opportunities to collect all the many Legendary and epic heroes  in this game for free just stick to it and grind  it out. Your going to get alot of sacred free shards along the way and save them for a X2 event

Your Epics and Legendary champs will come by more than just shard means

But I personally I find with summoning that it seems like there are times when some champions are  more pronounced to get from shards than others

If you see that just take a break and try again another time