The game mechanics and damage formula are far too (needlessly) complicated to understand without prolonged study. It's best to just read a guide rather than try to puzzle it out yourself, at least at first. I've made damage calculators that can aid in exploration, but even those rely on some degree of experience so you know what inputs to feed in. Give them the stats of LV1 characters with no weapon or eidolon grid and you'll get results that aren't representative of real situations.