Beating Normal and Hard Mode
Beating normal mode is one of the first barriers in the game to overcome. The following guides are written with hard mode in mind. If you read these they'll get you through normal mode, and as you progress through normal mode, they'll put you on the fastest path to beating hard mode.
Reading mythic roles will also help.
Beating Hell Mode
Getting from Hard Mode to Hell Mode is going to be much harder than getting good at Hard Mode. The following guide will explain how to work towards it while you get better at hard mode: How to Beat Hell Mode.
There are also unique bosses in this mode that can attack your board, killing or debuffing your units. You can see the full list of these bosses here.
Below I've embedded a demonstration of the physical damage hell mode strategy with an SB6 account.
Immortals
Should you unlock the new Immortal Rarity guardians? I would recommend first completing the unlock order guide. Once you do that, you have a lot more freedom in deciding how you want to progress in the game. Once you've completed that, check out the Immortal Guardians Guide.
In the meantime here's a showcase of the new Immortal Frog
