If you are like me, you are trying to hoard as many 5* shards as possible before stark spidey comes out. Unless it is a crazy good deal, I try not to spend money on this game. So here is a good way to gain 5* shards.
Take for example the lower tier champs, in this case it is Groot. I have no use for this character, so when he comes up in the basic arena we know he will not go for much. I did 2.2 mill both times and got him.
So I sell my 3/30 Groot for 425 5* shards, then I get him back in the arena as well as 300 more 5* shards and the rerank him back to 3/30. If you do this twice you have gained 1450 5* shards and you still get to keep your Groot.


Just a tip I thought I would share.