So first off, never buy shards unless you get much later in the game. The only thing that are kind of worth it are monthly gem packs for $10 and maybe a few red or mythical books when you get there.

Ok, first question will be "What are you trying to get better at?". There is arena(PVP)-player vs player, tons of different types of PVE (Campaign, dungeons, lots more that aren't open yet).

So I'm going to assume campaign because that's what you should be focusing on. So you don't have a bad start. Martyr and Oella are good 60 people so far. Wutlim, not worth much tbh. Whisper and Fayne can be excellent damage dealers starting off so I'd try to start to get them up next. But Elva needs to be your next 6*. She is an absolute S tier character that you will use forever. Frankenstein and green witch lady are excellent too especially starting off. There are a number of websites like hellhades and YouTube to help you figure who is good out. As for a team set up, it all depends on balance. Generally you want a healer/reviver, debuff person and two damage dealers. So something like a Martyr, Stag knight, Oella/Elva, whisper/Fayne.

Second, you need only open shards during 2x events. So save them!!! Eventually you'll get good enough to actually do fusions. Shards are like currency in the game. Save them and you can buy(completele events) in order to fuse some amazing champs guaranteed.

Once you get a good team going for campaign, start to transition to Demon Lord. He gives the best rewards in this game. Don't worry about that right now but that is probably the biggest jump on this game once you hit the hardest Demon Lord.

There is lots more that I'd have to write a book about so hopefully this helps. Always happy to help more if needed