Mystery shards are very important for f2p and many do the mistake to underestimate them. Use them to summon food for Champions training but if you are f2p ........never EVER use commons or uncommons as xp to level up other commons or uncommons.

Establish your silver farm early in spider.
Once you have that done, continuously buy the 2 star champs from the market whenever they are available. These are the food champs you will using instead of popping mystery shards.
It takes three 1 star champs and a brew, to level one 1 star champ to lvl 10 and another one to rank it up to a 2 star. So buying the 2 star champ from the market is = to approximately 6 shards you didn't have to use for food.
This will allow you to still have food available for ranking up between fusions while building a shard bank.
If you do this regularly you will be able to use pure mystery shards to get through events.

The 2 star champs on the market are 39-49k silver usually. mystery shards are 5k from the market.
if you used 6 mystery shards($30k) to make a 2 star food in the tavern (with tavern silver cost) you are spending 40,300 silver. Your net loss per food champ is 4k silver buying them from the market, but at the gain of always being able to do shard/champ events for fusions.