Just finished this one and had a blast! It's a tight resource optimization puzzle, and it can only be solved by learning the mechanics and understanding what a "good" result for each fight looks like. The resting mechanic rewards you for balancing risk and reward, routing the fights in an optimal order, and understanding which characters are needed for which fights. That last one requires more thought than in many RPGs since this very much isn't a rock-paper-scissors battle system, so there are genuine trade-offs with every decision that are more than "use character X to beat Y". Every mechanic has a purpose, and the best way through each battle isn't always the most obvious one. The end result is that you really need to understand the game to do well; a fight that "seemed okay" might not be good enough in the long run. New mechanics are introduced over time so that you're constantly introduced to something new before the old mechanics start to get anywhere near being repetitive (despite each fight being relatively long in itself).
While there's enough leeway that you don't need good RNG, you absolutely can run out of resources if you let mistakes stack up over time. It's worth taking the time to redo fights that went poorly and fix any mistakes.