Pretty sure it has already been said plenty of times, but the game balance for this is not in a good place at the moment. Fights as of the initial release end up being curb stomp battles either in or against your favor with little inbetween that isn't a joke to beat or a near impossible challenge. I presume that the current damage calculations are based off of raw stats determining how much damage is dealt and received on a 1:1 ratio which, while simple... leads to almost every fight being a check of who has the higher stats and better stat allocation. Adding a helper NPC in combat helps to alleviate this issue somewhat... but only so far as to spread the burden of attack, defense, and support across multiple characters as opposed to having only one do everything.
While I am unsure of exactly how to fix this issue, adapting the system so that way it is much harder to no-sell damage or have no chance of victory against more than a single slime in the beginning of the game before multiple level ups seems like a good place to start.