if you put money into nothing but more units for 3 or 4 upgrades it's not too bad. usually it's more a matter of figuring out which upgrade is the one you actually need, and any grind you put into other upgrades is painfully unhelpful.
generally speaking once you've got your unit headcount up (will be useful later + more xp now while units dying is your only source of xp) you gotta grind up blocker hp until you have a unit capable of taking a hit without dying immediately. once you have that, it should be fairly obvious what you need to do to get hits in (your blocker may even do this themselves), and once you're getting xp from getting hits in you're basically off to the races.
i had a data wipe fairly recently and replayed the game, and was able to beat each heroine in under the required amount of runs to get the bonus final boss, first try on each. the dumb slime priestess was the only one that felt particularly difficult (the change of pace is a nice idea but geez is it frustrating). it feels a lot like those "upgrade" flash games on kongregate back in the day, which i appreciate.