PSA: Not all content is meant to be easy
I get everyones frustrations at the start of every month when it feels like the whole world is against you and no matter what you do, you just can't clear the content. But, the harder content is exactly that. Harder. Not everyone is going to be able to walk through it without spending items and units. But that is the whole point of a progression based game. You can complain that players with bigger rosters have it easier, and they do, but they too were at your stage once struggling to complete anything. A better way to tackle these problems would be ask people for help. The community will help you, if you don't have a certain champ, we'll try to help you with someone you do have. If you absolutely can't, you have to learn to accept that that is your current ceiling and you will have to grind and improve your roster. Soon, you too will be able to walk through content. But for now, patience.